The Prostate Cancer Prevention Trial (PCPT)-a randomized placebo-controlled study of the efficacy of finasteride in preventing prostate cancer-offered the opportunity to prospectively study effects of finasteride and other covariates on the health-related quality of life of participants in a multiyear trial.We assessed three health-related quality-of-life domains (measured with the Health Survey Short Form-36: Physical Functioning, Mental Health, and Vitality scales) via questionnaires completed by PCPT participants at enrollment (3 months before randomization), at 6 months after randomization, and annually for 7 years. Covariate data obtained at enrollment from patient-completed questionnaires were included in our model. Mixed-effects model analyses and a cross-sectional presentation at three time points began at 6 months after randomization. All statistical tests were two-sided.For the physical function outcome (n = 16 077), neither the finasteride main effect nor the finasteride interaction with time were statistically significant. The effects of finasteride on physical function were minor and accounted for less than a 1-point difference over time in Physical Functioning scores (mixed-effect estimate = 0.07, 95% confidence interval [CI] = -0.28 to 0.42, P = .71). Comorbidities such as congestive heart failure (estimate = -5.64, 95% CI = -7.96 to -3.32, P < .001), leg pain (estimate = -2.57, 95% CI = -3.04 to -2.10, P < .001), and diabetes (estimate = -1.31, 95% CI = -2.04 to -0.57, P < .001) had statistically significant negative effects on physical function, as did current smoking (estimate = -2.34, 95% CI = -2.97 to -1.71, P < .001) and time on study (estimate = -1.20, 95% CI = -1.36 to -1.03, P < .001). Finasteride did not have a statistically significant effect on the other two dependent variables, mental health and vitality, either in the mixed-effects analyses or in the cross-sectional analysis at any of the three time points.Finasteride did not negatively affect SF-36 Physical Functioning, Mental Health, or Vitality scores.