Eighty patients with severe psoriasis were treated in a double-blind fashion with either an initial dose of 10 mg, 25 mg or 50 mg of etretin daily or with placebo. Follow-up examinations were carried out monthly and the efficacy of treatment was evaluated by using the PASI score. Adverse effects of the treatment were recorded monthly; liver enzymes, cholesterol and triglycerides were measured. After 2 months of treatment the maintenance dose was reduced in some of the patients either because of complete remission or adverse effects. After 2 months treatment, groups receiving 25 mg/day and 50 mg/day showed significantly lower PASI scores than the placebo group. The 10 mg/day group showed a response intermediate between the 25 mg and 50 mg groups and the placebo group. Thus, the optimal initial dose seems to be approximately 25 mg/day and the maintenance dose somewhat lower. Six months after the start of treatment there were no significant differences between the four groups; the last follow-up examination took place during the summer and some of the patients probably experienced spontaneous improvement. Although clinical adverse effects were frequent in all groups, severe side effects, namely hair loss and paronychia, occurred frequently only among patients treated with an initial dose of 50 mg of etretin daily. The effect of treatment on liver enzymes, cholesterol and triglycerides was minimal.