Background: Fractures of the femoral shaft in children are relatively uncommon but serious injuries that disrupt the lives of children and their carers and can result in significant long-term disability. Treatment involves either surgical fixation, such as intramedullary nailing or external fixation, or conservative treatment involving prolonged immobilisation, often in hospital. Objectives: To assess the effects (benefits and harms) of interventions for treating femoral shaft fractures in children and adolescents. Search methods: We searched the Cochrane Bone, Joint and Muscle Trauma Group Specialised Register (accessed 16 August 2013), the Cochrane Central Register of Controlled Trials (The Cochrane Library 2013 Issue 7), MEDLINE (1946 to August Week 1 2013), EMBASE (1980 to 2012 week 9), CINAHL (16 August 2013), clinical trials registries, conference proceedings and reference lists; and contacted trial authors and experts in the field. Selection criteria: Randomised and quasi-randomised controlled trials comparing conservative and surgical interventions for diaphyseal fractures of the femur in children under 18 years of age. Our primary outcomes were functional outcome measures, unacceptable malunion, and serious adverse events. Data collection and analysis: Two authors independently screened and selected trials, assessed risk of bias and extracted data. We assessed the overall quality of the evidence for each outcome for each comparison using the GRADE approach. We pooled data using a fixed-effect model. Main results: We included 10 trials (six randomised and four quasi-randomised) involving a total of 527 children (531 fractures). All trials were at some risk of bias, including performance bias as care provider blinding was not practical, but to a differing extent. Just one trial was at low risk of selection bias. Reflecting both the risk of bias and the imprecision of findings, we judged the quality of evidence to be 'low' for most outcomes, meaning that we are unsure about the estimates of effect. Most trials failed to report on self-assessed function or when children resumed their usual activities. The trials evaluated 10 different comparisons, belonging to three main categories. Surgical versus conservative treatment Four trials presenting data for 264 children aged 4 to 12 years made this comparison. Low quality evidence (one trial, 101 children) showed children had very similar function assessed using the RAND health status score at two years after surgery (external fixation) compared with conservative treatment (spica cast): mean 69 versus 68. The other three trials did not report on function. There was moderate quality evidence (four trials, 264 children, aged 4 to 12 years, followed up 3 to 24 months) that surgery reduced the risk of malunion (risk ratio (RR) 0.29, 95% confidence interval (CI) 0.15 to 0.59, 4 trials). Assuming an illustrative baseline risk of 115 malunions per 1000 in children treated conservatively, these data equate to 81 fewer (95% CI 47 to 97 fewer) malunions per 1000 in surgically-treated children. Conversely, low quality evidence indicated that there were more serious adverse events such as infections after surgery (RR 2.39, 95% CI 1.10 to 5.17, 4 trials). Assuming an illustrative baseline risk of 40 serious adverse events per 1000 for conservative treatment, these data equate to 56 more (95% CI 4 to 167 more) serious adverse events per 1000 children treated surgically. There was low quality evidence (one trial, 101 children) of similar satisfaction levels in children and parents with surgery involving external fixation and plaster cast only. However, there was low quality evidence (one trial, 46 children) that more parents were satisfied with intramedullary nailing than with traction followed by a cast, and that surgery reduced the time taken off from school. Comparisons of different methods of conservative treatment The three trials in this category made three different comparisons. We are very unsure if unacceptable malunion rates differ between immediate hip spica versus skeletal traction followed by spica in children aged 3 to 10 years followed up for six to eight weeks (RR 4.0, 95% CI 0.5 to 32.9; one trial, 42 children; very low quality evidence). Malunion rates at 5 to 10 years may not differ between traction followed by functional orthosis versus traction followed by spica cast in children aged 5 to 13 years (RR 0.98, 95% CI 0.46 to 2.12; one trial, 43 children; low quality evidence). We are very unsure (very low quality evidence) if either function or serious adverse events (zero events reported) differ between single-leg versus double-leg spica casts (one trial, 52 young children aged two to seven years). Low quality evidence on the same comparison indicates that single-leg casts are less awkward to manage by parents, more comfortable for the child and may require less time off work by the caregiver. Comparisons of different methods of surgical treatment The three trials in this category made three different comparisons. Very low quality evidence means that we are very unsure if the rates of malunion, serious adverse events, time to return to school or parental satisfaction actually differ in children whose fractures were fixed using elastic stable intramedullary nailing or external fixation (one trial, 19 children). The same applies to the rates of serious adverse events and time to resume full weight-bearing in children treated with dynamic versus static external fixation (one trial, 52 children). Very low quality evidence (one trial, 47 children) means that we do not know if malunion, serious adverse events and time to resume weight-bearing actually differ between intramedullary nailing versus submuscular plating. However, there could be more difficulties in plate removal subsequently. Authors' conclusions: There is insufficient evidence to determine if long-term function differs between surgical and conservative treatment. Surgery results in lower rates of malunion in children aged 4 to 12 years, but may increase the risk of serious adverse events. Elastic stable intramedullary nailing may reduce recovery time. There is insufficient evidence from comparisons of different methods of conservative treatment or of different methods of surgical treatment to draw conclusions on the relative effects of the treatments compared in the included trials.