Fifty-four acutely manic patients were allocated to treatment on a double-blind basis with either carbamazepine or lithium carbonate. The short-term effects of treatment were studied over a period of six weeks and the longer term, prophylactic, effects over a period of up to a year. Additional 'rescue' medication was allowed when clinically indicated. There was a high drop-out rate from the trial. Despite this, it appeared that valid comparisons between the two treatments could be made. No statistically significant differences were found, but carbamazepine appeared slightly less effective as a treatment for acute mania and more effective as a prophylactic treatment in this group of patients. Possible predictors of individual responsiveness to each treatment are discussed.