To confirm that plasma delta 4 androstenedione (delta 4) is the main precursor for 5 alpha-androstane-3 alpha, 17 beta-diol glucuronide (Adiol G) in patients with idiopathic hirsutism (IH), delta 4 was cutaneously applied to five normal women and five women with IH. Several parameters of androgen metabolism were assayed basally and throughout the studies. Those included plasma delta 4, testosterone, and dihydrotestosterone as well as urinary Adiol G and testosterone glucuronide excretion. Under basal conditions plasma testosterone, delta 4, and dihydrotestosterone did not differ significantly between the two groups of subjects. Urinary Adiol G excretion was significantly higher (P less than 0.01) in IH patients [123 +/- 36 (SE) micrograms/24 h] than in the normal women group (45 +/- 20 micrograms/24 h). After percutaneous administration of delta 4, plasma delta 4 increased in both groups by nearly 600% and there was a 300% increase in Adiol G excretion in IH patients (336 +/- 57 micrograms/24 h), whereas only a 50% increase occurred in normal women (65 +/- 17 micrograms/24 h). We postulate that plasma delta 4 may be the main precursor accounting for the increased production of urinary Adiol G in women with IH, in whom hirsutism may be due to a high 5 alpha-reductase activity. Indeed, 5 alpha-reductase as measured in vitro in pubic skin was significantly higher in hirsute patients (224 +/- 66 fmol/mg skin X h) than in normal women (45 +/- 15 fmol/mg skin X h).