The results of a phase II clinical trial investigating the monoamine oxidase (MAOA) inhibitor phenelzine in biochemically recurrent, hormone-sensitive prostate cancer have been published. In this open-label, single-arm clinical trial, 20 patients were recruited to receive 30 mg of phenelzine orally twice daily. The primary end point was the proportion of patients achieving a serum PSA decline of ≥50% from baseline and mood symptoms were assessed using the hospital anxiety depression score questionnaire. The results showed that 10% of patients had a maximal PSA level decline of ≥50% and 25% of patients showed declines of ≥30%. At 12 weeks after commencing treatment, 17 patients were still receiving phenelzine. Of these men, 20% had PSA declines of ≥30% and 5% had declines of ≥50%. Questionnaire results showed that anxiety decreased but depressive symptoms did not change. Rare, considerable but reversible cardiovascular toxic effects were observed; however, most treatment-related toxic effects were mild. Thus, MAOA inhibitors are a promising treatment option for men with recurrent, hormone-sensitive prostate cancer.