Objective The aim of this study was to investigate the effects of hormone therapy (HT) on depression and depressive symptoms in prostate cancer patients undergoing 6 months of HT. Methods One hundred two prostate cancer patients who had been prescribed HT completed the Zung Self‐rating Depression Scale (SDS) and two questions about their sexual enjoyment and performance, plus a background questionnaire before HT, after 8 to 10 weeks of HT and again after 16 to 20 weeks of HT. Results There was a significant increase in SDS scores from before to during HT. High depression score before HT was a significant predictor of later increases in depression during HT. Increases in depressive symptoms were restricted to 8 of the 20 SDS symptoms, the most powerful change being in sexual anhedonia, which was a result of decreased ability to perform during sexual activity. Conclusions The association between HT and elevated depression is confirmed, but the relative influence of sexual anhedonia over other depressive symptoms expands the understanding of this association. The effects of decreased ability to perform during sex appear to dominate the increase in depression during HT. Copyright © 2016 John Wiley & Sons, Ltd.