The metabolism of bone marrow cells during the development of acute promyelocytic leukaemia has only been scarcely characterized, even though such knowledge might improve our understanding of the mechanisms of leukemogenesis as well as of drug treatment failure. We have investigated the in vitro oxygen consumption and the metabolism of palmitate in rat bone marrow cells during development of acute promyelocytic leukaemia. As the leukaemia progressed, the cellular oxygen consumption, the β-oxidation of palmitate and the incorporation of palmitate into phospholipids all increased markedly. Cyclophosphamide supplement led to a temporary reduction of the palmitate metabolism, but did not lower the increased oxygen consumption. We conclude that the cellular metabolic rate is elevated during the progression of acute promyelocytic leukaemia, and that this might reflect an enhanced proliferative rate of the malignant cells.