A 12-month double-blind trial of carbamazepine vs lithium, given as sole treatment for the prophylaxis of bipolar affective disorder, was carried out in 31 patients. All were previously stable on lithium; 15 were switched over to carbamazepine and 16 remained on lithium. Although the overall relapse rate was similar in the 2 groups (6 on carbamazepine, 8 on lithium), nearly all the relapses in carbamazepine occurred in the first month, probably precipitated by lithium withdrawal. Two patients on carbamazepine developed a rash and were withdrawn. More side effects were noted during the early stages on carbamazepine. Patients on lithium tended to gain weight (+4 kg) compared with carbamazepine (-3.1 kg). It is concluded that carbamazepine is as effective as lithium in the prophylaxis of bipolar affective disorder; changeover from lithium to carbamazepine should be done slowly.