Acedapsone (4,4′-diacetyldiaminodiphenyl sulfone) is a long-acting intramuscular repository derivative of dapsone. Acedapsone is mainly used as a depot leprostatic agent. The parent compound possesses little activity against M. leprae but is metabolized into active dapsone. Its half-life is 46 days, and that of the derived dapsone is 43 days. A 300-mg intramuscular dose maintains dapsone levels in volunteers above the inhibitory concentration for M. leprae for approximately 100 days. Microbiologic and clinical responses are somewhat slower than those for daily dapsone are. Long-term studies with acedapsone by injection five times yearly have yielded encouraging results. Acedapsone shows promise especially in regions where, or in patients in whom, long-term oral therapy is not practical.