PURPOSE The microgranular variant (M3v) of acute promyelocytic leukemia (APL) rarely has been reported in a pediatric series of acute nonlymphoblastic leukemia (AnLL). We reviewed the clinical and biologic features of childhood M3v cases in our AnLL series. PATIENTS AND METHODS From January 1970 to January 1991, 11 children with M3v were admitted and treated at our center. A diagnosis was made according to French-American-British (FAB) criteria. Morphologic examination, cytochemical analysis, and immunophenotyping were performed by a single pathologist. From January 1984, the diagnosis was confirmed by cytogenetic and, subsequently, by molecular analysis on frozen material. RESULTS In our series, the overall incidence of children with APL was unusually high, 31.2% of the AnLL and M3v constituted one case in every four cases of APL. Even restriction of the analysis to the time when either cytogenetic and DNA studies confirmed the diagnosis, the incidence did not change. The immunophenotype of M3v cases was identical to that described for the hypergranular type, but an unexpected association of CD2 with M3v was shown. The onset was characterized by marked hyperleukocytosis (median WBC count, 87 x 10(9)/L) unlike classic APL. Disseminated intravascular coagulation (DIC) was always present and severe. Hyperleukocytosis and DIC were responsible for the high incidence of deaths for hemorrhagic events in the first days after onset (eight of 11 patients). CONCLUSIONS In our experience, for unknown reasons, M3v may occur in childhood more than generally was considered. The clinical course and prognosis seem worse in M3v than in typical APL cases.