Among 17 patients with non acute monocytic (CMoL) and myelomonocytic leukemia (CMML) one patient presented with atypical findings and an unusual clinical course. Prominent massive pancytopenia had occurred ever since the diagnosis was established. To our knowledge this symptom has not been described in cases with CMML and CMoL. Splenectomy afforded temporary improvement of the anemia but induced sustained, marked thrombocytosis. An unusually prolonged survival time of 44 months was achieved, presumably by the continuous, careful replacement of blood, the splenectomy and the relinquishment of cytotoxic therapy. A gradual, continuously developing blastic crisis and finally the transition to an acute leukemia were observed.