A 30-year-old woman was diagnosed as having an acute promyelocytic leukaemia in September 1981. Chemotherapeutic courses of daunomycin, ara-C, thioguanine and prednisolone were administered, resulting in a complete remission. A relapse occurred in January 1982, and chemotherapy did not lead to a second complete remission; neutropenia persisted with a marked left shift of the marrow granulopoiesis. Courses of chemotherapy were given throughout the study. In September 1982, marrow promyelocytes markedly increased, making up 51% of the nucleated cells. One month later the situation was unchanged, and 13-cis-retinoic acid (1 mg/kg) was administered by mouth. Gradually the marrow proportion of promyelocytes decreased to normal levels. The peripheral blood and marrow were still normal after 20 weeks of treatment with retinoic acid. Thus, retinoic acid seemed to have been inducing differentiation in an abnormally increased, maturation-deficient population of promyelocytes in a patient with acute promyelocytic leukaemia.