A 28-year-old woman in the third trimester of her pregnancy was found to have acute myelocytic leukemia. The baby was delivered by Cesarean section and her leukemia underwent spontaneous remission. However, 3 months later, she presented with massive painful leukemic infiltration of the breasts as initial manifestation of relapse, followed by systemic symptoms of leukemia. In vitro culture of the leukemic cells demonstrated characteristics of macrophage cell line. This case illustrates a unique sequence of events: spontaneous remission after the termination of pregnancy, which has profound hormonal alterations, and relapse in a very hormone-sensitive organ, the breast, a few months later when the hormonal milieu was resumed. This suggests hormonal dependence of her leukemic cells and potential for hormonal manipulation in a certain subset of human leukemia.