The prophylactic efficacy of carbamazepine slow release (CBZ) at ke different blood levels and lithium carbonate slow release (LI) was compared in a retrospective/prospective, randomized, 2-year open trial. 84 patients with a DSM-III-R diagnosis of recurrent affective disorder who had no prophylactic medication in the 2 years preceding the trial (no LI nonresponders), were randomly allocated to three treatment groups: CBZ low (15-25 mumol/l), CBZ high (28-40 mumol/l) and LI (0.6-0.8 mumol/l). Fifty-eight patients completed the full observation period of 2 years, 26 patients dropped out. There were no statistically significant differences in the efficacy of the prophylactic treatment for bipolar patients. For the unipolar patients, the group with a low CBZ serum level showed no reduction in the duration of episodes. The two other treatment groups seem to be equal in attenuation of a unipolar course of an affective disorder.