One hundred fifty patients with treatment-resistant nodulocystic acne were entered into a double-blind clinical study. Three different dosing levels (0.1, 0.5, 1.0 mg/kg/day) were used in equal-sized groups. In addition to the clinical response, the clinical side effects, the laboratory abnormalities, and the duration of the induced remissions were evaluated with each dose of the drug. There was a highly significant clinical response to treatment with all three dosages of isotretinoin. There was no significant difference in the clinical response between dosages. However, 42% of the patients who received 0.1 mg/kg/day of isotretinoin required retreatment with the drug. This finding, coupled with only minor differences in the clinical side effects and the laboratory abnormalities, indicates that higher dose levels of isotretinoin are indicated for treatment of nodulocystic acne.