The therapeutic effect of the alpha 2-antagonist yohimbine in erectile dysfunction was studied in a double-blind placebo-controlled design. Thirty-one male patients underwent extensive clinical, urological, and psychiatric diagnosis and were dichotomically classified into an organic and a nonorganic subgroup. Following a 1-week placebo run-in period, patients were randomly assigned to a placebo or a verum group (yohimbine 15 mg daily) for a treatment period of 7 weeks. The Clinical Global Impression (CGI) scale was used as the primary efficacy parameter. Additionally, nocturnal penile tumescence and rigidity (NPTR) were measured. Global assessment of erectile function applying the CGI scale revealed, beyond a placebo effect in both organic and nonorganic patients, a therapeutic effect in the subgroup of nonorganic patients, with a significantly greater improvement in the yohimbine group compared to the placebo group. No superiority of yohimbine compared to placebo was found in the organic patients. These findings on the subjective level had no correlate in the NPTR recordings. The NPTR parameters were unchanged under yohimbine treatment in both the nonorganic and organic subgroup. No interrelation was found between subjective improvement and NPTR alterations. Polysomnographic control of the NPTR registrations ensured that the duration of REM sleep under treatment was not influenced.