RNA transcriptional levels of the proto-oncogenes c-sis, c-fos, c-myb and c-myc were measured in peripheral blood leukemic blast cells of 16 patients with acute myeloid leukemia (AML) of different FAB subtypes, 8 being at diagnosis and 8 upon relapse. The studied proto-oncogenes were found to be regulated but varied considerably within morphologically identical subtypes. This is consistent with the clinically observable diverse behavior of seemingly identical AMLs as to the course and outcome of the individual disease. Overexpression of c-sis and c-myc was found more often in AML upon relapse than at diagnosis and in two cases overexpression not found at diagnosis was present at relapse. This implies alterations of biological behavior in the course of antileukemic drug therapy. A decline of c-myb expression was observed in one patient studied throughout therapy which was found to be associated with a complete but transient hematological remission after chemotherapy.