The authors report a de novo AML (M2) patient associated with 5q- as the sole karyotypic abnormality. A 76-year-old woman was referred to our hospital because of anemia and leukocytosis. On examination a neck lymph node was enlarged, but neither the liver nor the spleen could be palpated. The hemoglobin level was 7.1g/dl, the mean corpuscular volume 102fl and the white-cell count was 256.1 x 10(3)/microliters with 87% blast cells. The platelet count was 10.9 x 10(4)/microliters. The bone marrow was hypercellular with 79.8% blast cells and showed dysmegakaryocytopoietic features (hypolobulation, multiple separated nuclei and micromegakaryocytes). Blast cells gave a positive reaction for peroxidase and alpha NB esterase which was not blocked by NaF. The diagnosis of AML (M2) was made but she died before chemotherapy. Autopsy revealed general hemorrhagic tendency and leukemic cell infiltration. Chromosome analysis of the bone marrow showed 46,XX,del(5) (q13q31). Electron micrographs revealed increase of micromegakaryocytes as small as myelocytes and aggregation of demarcation membranes in some megakaryocytes. This may suggest that some molecular changes, instead of karyotypic evolution, contributed to a leukemic transition from the 5q- syndrome to AML with 5q- as the sole abnormality.