In three patients with trazodone-resistant unipolar depression, carbamazepine 400 mg/day was co-administered for 4 weeks. They all showed favourable responses; two non-delusional patients completely or almost recovered, while one delusional patient showed considerable improvement in depressive symptoms other than delusions of guilt. Plasma concentrations of trazodone and its active metabolite m-chlorophenylpiperazine (m-CPP) were decreased, while the m-CPP/trazodone ratio was increased in all cases. The present report suggests that carbamazepine augments antidepressant effects of trazodone by an enhancement of serotonin function.