Although obesity at the time of prostatectomy has been associated with prostate cancer recurrence, it is unknown whether obesity before or after surgery, or weight change from the years prior to surgery to after surgery is associated with recurrence. Thus, we examined the influence of obesity and weight change on recurrence after prostatectomy. We conducted a retrospective cohort study of 1,337 men with clinically localized prostate cancer who underwent prostatectomy performed during 1993-2006 by the same surgeon. Men self-reported weight and physical activity at 5 years before and 1 year after surgery on a survey during follow-up. Mean follow-up was 7.3 years. We estimated multivariable-adjusted HRs of prostate cancer recurrence comparing obesity at 5 years before and at 1 year after surgery with normal weight, and a gain of more than 2.2 kg from 5 years before to 1 year after surgery with stable weight. During 9,797 person years of follow-up, 102 men recurred. Compared with men who had stable weight, those whose weight increased by more than 2.2 kg had twice the recurrence risk (HR = 1.94; 95% CI, 1.14-3.32) after taking into account age, pathologic stage and grade, and other characteristics. The HR of recurrence was 1.20 (95% CI, 0.64-2.23) and 1.72 (95% CI, 0.94-3.14) comparing obesity at 5 years before and at 1 year after surgery, respectively, with normal weight. Physical activity (≥ 5 h/wk) did not attenuate risk in men who gained more than 2.2 kg. By avoiding weight gain, men with prostate cancer may both prevent recurrence and improve overall well-being.