Objective: Sleep disturbances are common among veterans with chronic military-related posttraumatic stress disorder (PTSD). This article reports the results of a multicenter clinical trial that explored the clinical correlates of reported sleep impairment in these veterans and tested the impact of the second-generation antipsychotic risperidone upon these symptoms. Method: This article reports secondary analyses of a 24-week multicenter randomized placebo-controlled trial of adjunctive risperidone in patients with chronic military-related PTSD symptoms (n = 267, 97% male) who were symptomatic despite treatment with antidepressants and other medications. The study was conducted between February 2007 and February 2010. DSM-IV PTSD diagnoses were made by using the Structured Clinical Interview for DSM-IV-TR Axis I Disorders, Nonpatient Edition. Sleep disturbances were assessed principally by using the Pittsburgh Sleep Quality Index (PSQI) (primary outcome measure). Analyses were conducted using bivariate correlations and longitudinal mixed model regressions. Results: Eighty-eight percent of the patients in this study had clinically significantly impaired sleep on the PSQI. Severity of sleep disturbances correlated with PTSD symptom severity as measured by the Clinician-Administered PTSD Scale (CAPS) and reductions in multiple measures of quality of life (Veterans RAND 36-item Health Survey [SF-36 V] subscales, Boston Life Satisfaction Index). Risperidone produced small but statistically significant effects on total PSQI scores (main effect of drug: F1,228 = 4.57, P = .034; drug-by-time interaction: F2,421 = 4.32, P = .014) and severity of nightmares as assessed by the CAPS (main effect of drug: F1,248 = 4.60, P = .033). The improvements in sleep quality produced by risperidone correlated with reductions in PTSD symptom severity and improvement in the mental health subscale of the SF-36 V. Conclusions: This study highlighted the near universality and significant negative impact of severe disturbances in sleep quality in veterans with chronic militaryrelated PTSD who were partial responders to standard pharmacotherapies. The modest improvements in sleep quality produced by adjunctive risperidone were correlated with limited reductions in PTSD severity and improvements in quality of life.