This prospective study examined the incidence of mania or hypomania in 230 patients with recurrent depression treated with imipramine. Overall, only six individuals (2.6%) developed hypomania, representing 0.9% of those in the acute phase and 2.5% of those in the continuation phase of drug treatment. Patients with a history of bipolar II depression (N = 33) did not have a greater incidence of hypomania than those with unipolar depression (N = 197). Younger patients did not switch to hypomania more rapidly than older ones, and women were not more likely to switch than men. Systematic assessment of mania, stringent diagnostic criteria, and the recurrent nature of the sample may account for this low incidence of hypomania compared to that reported by other investigators.