Mid-life onset male dysthymic disorder (DD) seems to be a distinct clinical condition with limited therapeutic options. Testosterone replacement is mood-enhancing and has been proposed as an antidepressant therapy, though this strategy has received limited systematic study. We therefore conducted a six-week double-blind placebo-controlled clinical trial in 23 men with DD and with low or low-normal testosterone (T) level (i.e, screening total serum testosterone <350 ng/dL). Enrolled men were randomized to receive intramuscular injections of 200 mg of testosterone cypionate or placebo every 10 days. The primary outcome measures were the Clinical Global Impression (CGI) improvement score and the 21-item Hamilton Depression Rating Scale (HDRS) score.Twenty-three patients were randomized. The mean (SD) age of the enrolled patients was 50.6 (7.0) years and that of total testosterone level was 339 (93) ng/dL. The median duration of the current dysthymic episode was 3.6 (2.3) years, and the mean (SD) HDRS was 14.0 (2.9). After the intervention, the mean HDRS score decreased significantly more in the testosterone group (7.46 [4.56]) than in the placebo group (1.8 [4.13], t21 = -3.07, P = 0.006). Remission, defined as a CGI improvement score of 1 or 2 and a final HDRS score lower than 8, was achieved by 7 (53.8%) of 13 in the testosterone group and 1 (10%) of 10 in the placebo group (P = 0.03). Testosterone replacement may be an effective antidepressant strategy for late-onset male dysthymia.