Sexual performance was evaluated in 28 men, thirty to sixty years of age, with functioning renal allografts: 13 were potent (controls), 10 had moderate sexual dysfunction, 5 had marked sexual dysfunction. Penile blood pressures, serum hormone levels, plasma zinc levels, and penile venous angiography were evaluated in search of causes of impotence. Thirteen of 15 impotent transplant patients (87 per cent) had marked abnormalities in at least one of the four areas studied. Systematic search for etiologic factors may permit specific therapy for impotence, which occurred in 54 per cent of the 28 kidney transplant patients analyzed.