Late onset dysthymic disorder (DD) in middle-aged and elderly men responds poorly to established antidepressants. Previous studies noted an improvement in mood accompanying sildenafil citrate treatment for erectile dysfunction. We sought to evaluate whether sildenafil’s mood effects were independent of the effect on erectile function. A 6-week open label study was conducted with 20 male participants, aged 41–60 who were diagnosed with DD and who had normal erectile function. Participants were treated with sildenafil citrate 25 mg per day for 6 weeks. The primary outcome measure was the 21-item Hamilton Depression Rating Scale. Depressive and sexual symptoms were also evaluated using self-report questionnaires. Treatment with sildenafil resulted in a significant reduction in Hamilton Depression Rating Scale mean scores: from 14.61 ± 3.5 at baseline to 6.39 ± 5.13 at end of study (F(3,51) = 32.52, p ≤ 0.001). No changes in sexual functioning were detected. Significant improvement was also noted on the self-report measures of depressive symptoms. Sildenafil citrate might have an antidepressant effect on late onset DD, that is not attributable to improvement in erectile function. Possible explanations for this effect are offered. Larger placebo controlled studies are warranted.