The 5 q deletion is frequently found in myelodysplastic syndromes and acute non lymphoid leukemia, but this anomaly is usually found in secondary diseases and associated with many other chromosomal aberrations. This report describes four cases of "de novo" acute leukemia with a sole 5q- anomaly. They had no cytological, genetic or clinical characteristics of secondary disorders. It is important to note that of the four patients studied, three had proliferation of immature blast cells. One case was classified as a MO AML and two as "undifferentiated" acute leukemia. Furthermore, these four cases of acute leukemia showed a deletion of the same portion of the long arm of chromosome 5: q22q33. On the same part of this chromosome many hematopoietic growth factor genes have been located, like IL3 and GM-CSF which have early undifferentiated hematopoietic stem cells as a their target.