The iron-containing protein cytochrome P-450 is present in high concentration in the adrenal cortex and is involved in the synthesis of corticosterone. This study was designed to determine the cortisol response to adrenocorticotropin (ACTH) in patients with severe iron deficiency. Eleven patients with iron deficiency and 15 normal controls were studied. Fasting blood samples were taken from all the subjects before and 30, 60 and 120 min after infusion of 25 units of ACTH for plasma cortisol determination. Six patients had blood samples collected at night, too. The same test was performed in 6 patients with iron deficiency, 7 days after therapy with 800 mg of ferrous sulfate. No significant differences were observed between patients and controls for the baseline cortisol values. The cortisol secretion and the increment at 30, 60 and 120 min after ACTH infusion were significantly lower in patients than in controls, either before or after ferrous sulfate therapy. There were no significant differences between baseline and stimulated cortisol values in patients before and after 7 days of ferrous sulfate therapy. There was no change in cortisol secretion rhythm in patients with iron deficiency (cortisol level at night = 5.1 +/- 4.3 micrograms/dl). In conclusion, the results of the present study showed that, in patients with severe iron deficiency, the cortisol secretion after ACTH stimulation was decreased.