Twelve patients (eight men and four women) suffering from manic depressive disorder who were receiving both lithium and antipsychotic medication had their antipsychotic changed to depot haloperidol decanoate, with their lithium medication remaining unchanged. Successful transfer to haloperidol decanoate was achieved with a reduced incidence of extrapyramidal side effects, and 100% compliance in eight patients who all remained free of hypomanic relapse, including four patients with rapid-cycling disorder. By comparison, in the three years before switching to haloperidol decanoate these four patients had required sixteen admissions for hypomanic relapses resulting in thirty-four months in-patient treatment.