The incidence of carbamazepine-associated behavioral side effects in 65 individuals with mental retardation and additional seizure and/or psychiatric or behavioral disorders was evaluated. We identified 6 patients (9.2%) who experienced medication side effects, ranging from irritability to mania. Four of the 20 patients (20%) who received carbamazepine purely for treatment of a behavioral or psychiatric disorder experienced medication side effects, whereas none of the 21 patients treated for an isolated seizure disorder experienced similar effects. This difference was statistically significant, p less than .05. The incidence of behavioral side effects of medication was not associated with age, sex, or serum carbamazepine level. The chemical structure and mechanism of carbamazepine use in various disease processes were discussed.