The aim of the present study was to investigate cortisol levels under basal conditions and in response to ACTH stimulation in male patients with glucose-6-phosphate dehydrogenase (G-6-PD) deficiency. The study included 14 male controls and 12 patients with G-6-PD deficiency matched for age and race. Fasting blood samples were taken from all the subjects at rest, and 30, 60 and 120 min after the infusion of 0.25 mg of corticotropin for cortisol determination. The mean cortisol levels observed in the first hour after ACTH stimulation in the G-6-PD-deficient patients were significantly (p = 0.03) lower than in the control group. No significant differences were observed between patients and controls at rest, and in the second hour after stimulation. These data suggest that, in the adrenals, G-6-PD plays a role in the initial phase of cortisol production. However, 1 h after ACTH stimulation, G-6-PD probably is no longer rate limiting in the production of cortisol.