The hematological effects of carbamazepine, a drug effective in the acute and prophylactic treatment of manic-depressive disorder, were assessed in 43 subjects with affective disorder. Carbamazepine was found to cause statistically significant, but clinically insubstantial, decreases in white blood cell indexes. The consistent decreases in white cell counts observed with carbamazepine should be differentiated from the extremely rare, idiosyncratic blood dyscrasias reported in the literature.