In the period from August 1991 to August 1994, the Dutch Slide Review Committee of Adult Leukemias classified 14 leukemias as AML-M0. We reviewed the clinical characteristics and response to therapy of these patients. Eight patients were male. Patients' age ranged from 7 to 77 years (medium age 62 years). There was a striking homogeneity in morphological appearance of the blasts, being small to medium-sized round cells with often an eccentric nucleus with fine chromatin, several distinct nucleoli, and a high nucleo-cytoplasmic ratio. In addition to myeloid-associated markers such as CD13 and CD33, the blasts of all patients were positive for CD34 and HLA-DR, pointing to their immature differentiation stage. TdT was present in the blasts of 71%, CD7 was positive in the blasts of 42% of the patients. No consistent cytogenetic abnormalities were found. With respect to the treatment outcome, four patients achieved a complete remission after remission-induction treatment. The median survival was 4.5 months. Our present study shows AML-M0 to be an immature leukemia, uniform in morphology and immunological phenotype, with no consistent cytogenetic phenotype and with a poor clinical outcome.