Forty-one out-patients with a history of at least three attacks of depressive illness were randomly allocated to treatment on a double-blind basis for one year with either mianserin 20 mg three times daily plus placebo lithium tablets, or to lithium tablets once daily plus placebo mianserin tablets. After one year, the dosage of mianserin was increased to 30 mg t.d.s. for a further six months. All but three of the patients had previously been stabilized on prophylactic lithium therapy. Lithium was found to be significantly superior to mianserin in avoiding admission to hospital or ECT. The overall affective morbidity index, calculated from global rating, showed no significant difference between drugs, but the index of the mianserin group was higher in the second six months than in the first. The lithium group showed no such change. Lithium remains the choice for the prophylaxis of unipolar recurrent depressive illness.