Background: Computerized, tailored interventions have the potential to be a cost-effective means to assist a wide variety of individuals with behavior change. This study examined the effect of computerized tailored physical activity reports on primary care patients' physical activity at 6 months. Design: Two-group randomized clinical trial with physicians as the unit of randomization. Patients were placed in the intervention (n=187) or control group (n=207) based on their physician's assignment. Setting/participants: Primary care physicians (n=22) and their adult patients (n=394) from Philadelphia PA. The study and analyses were conducted from 2004 to 2010. Intervention: The intervention group completed physical activity surveys at baseline, 1, 3, and 6 months. Based on their responses, participants received four feedback reports at each time point. The reports aimed to motivate participants to increase physical activity, personalized to participants' needs; they also included an activity prescription. The control group received identical procedures, except that they received general reports on preventive screening based on their responses to preventive screening questions. Main outcome measures: Minutes of physical activity measured by the 7-Day Physical Activity Recall interview at 6 months. Results: Participants were 69% female, 59% African-American, and had diverse educational and income levels; the retention rate was 89.6%. After adjusting for baseline levels of activity and gender, there were no differences in physical activity at 6 months. The intervention group increased their total physical activity by a mean of 139 minutes; the control group had a mean increase of 109 minutes (p=0.45). Conclusions: Although physical activity increased within both groups, computerized tailored physical activity reports did not significantly increase physical activity between groups at 6 months among ethnically and socioeconomically diverse adults in primary care.