Eighteen patients with rapid cycling bipolar affective disorder were recruited for an open trial of carbamazepine. Of these patients all but 2 had been resistant to lithium prophylaxis. Twelve of the patients were able to complete at least 6 months on carbamazepine. Of these 12 patients, 2 had a complete remission of their affective disorders on carbamazepine alone, 2 completely responded to combined lithium and carbamazepine treatment, 3 had a slight beneficial effect from the drug, and for the other 5 patients carbamazepine was of no therapeutic benefit. This suggests that, while carbamazepine is effective for some rapid cycling patients, for the majority alternative treatment strategies are still required.