OBJECTIVE To find out indications for predicting MDS transforming into AML. METHODS Serum levels of human calcitonin (hCT) and calcitonin gene-related peptide (CGRP) were measured in 45 patients with myelodysplastic syndromes (MDS), 40 with acute myeloid leukemia (AML) and 50 controls by radioimmunoassay (RIA). Some patients in RA or RAEB stages were followed up to study changes in serum hCT and CGRP levels during clinical evolution. RESULTS hCT and CGRP were of high level in the sera of patients with MDS and de novo AML, which were significantly different from those of the controls (P < 0.01), and were higher in RAEB group than in RA group. hCT and CGRP levels increased gradually as RA was evoling into RAEB or RAEB was evolving into AML. CONCLUSIONS The serum levels of hCT and CGRP can be used as indices for the early prediction of MDS evolving into AML.