Isotretinoin, used to treat severe acne, has been shown to induce hormonal changes, especially to reduce 5 alpha-reductase in the production of the tissue-derived dihydrotestosterone (DHT) metabolite 3 alpha-Adiol G. However, the effects of isotretinoin on other pituitary, adrenal or gonadal hormones have not been thoroughly elucidated. In the present study, isotretinoin administered at a dose of 0.5 mg/kg/day for 4 weeks caused no marked changes in the serum levels of pituitary, adrenal or gonadal hormones or 3 alpha-Adiol G in patients with severe papulopustulotic acne (n = 19). After 12 weeks of therapy, there was a decrease in the levels of the precursor androgens androstenedione, testosterone and 3 alpha-Adiol G in 6/9 patients. Acne improved after 4.5 months in all but 2 male patients, who had very low serum hormone binding globulins (SHBG) and a high free androgen index (FAI). Isotretinoin did not affect the elevated LH/FSH ratio in a patient with the polycystic ovarian syndrome (PCOS); nor did it change the high FAI or low SHBG in the male patients. For comparison, tetracycline had no effects on the serum hormonal levels of patients with mild acne (n = 19) after 7 days of treatment. This study confirms that the effects of isotretinoin on the serum hormone levels are small and unlikely to be of relevance for the resolution of acne or the suppression of sebum excretion.