Seventy-four cases of pure acute monoblastic leukemia (AMol) have been retrospectively studied. All patients were treated at Hospital Saint-Louis between 1970 and 1978. Diagnosis was based on morphological and cytochemical features according to the FAB classification. This type of leukemia occurred at any age and in both sexes, with a high frequency of extramedullary involvements. Hyperleukocytosis was very frequent and was significantly correlated with increased blood and urine levels of lysozyme, with renal failure and hypokalemia, and with coagulation abnormalities. AMol still has a poor prognosis, despite a best remission rate (75%) obtained with rubidazone, since the duration of complete remission was short. Central nervous irradiation prolonged remission and prevented meningeal relapses, while 6 meningeal relapses occurred in the patients not irradiated. The high frequency of the extramedullary relapses, including gum and skin, emphasized the question of persistant blast cell sanctuaries after achievement of bone marrow remissions. A more intensive induction with several drugs active against monoblasts could be more efficient and prolong the duration of complete remissions.