We performed Southern blot analysis of the p53 gene in 62 patients (37 de novo myelodysplastic syndromes (MDS), of which 10 were studied after progression to acute myeloid leukemia (AML); 14 MDS secondary to chemo or radiotherapy; 11 de novo AML). Thirteen of the 56 patients studied cytogenetically had monosomy for the short arm of chromosome 17 and, in another patient who had secondary MDS, a translocation involving a breakpoint in 17p13 where the p53 gene was mapped was found. This patient was the only individual in whom a rearrangement of p53 DNA was seen. Sixteen of the 62 patients were studied by Northern analysis, and reduced or undetectable 2.8 kb p53 transcript was found in 6 of them, who had predominantly monosomy for 17p or chronic myelomonocytic leukemia. Rearrangements of the 53 gene, identifiable by Southern analysis, are a rare finding in patients with MDS and AML, even in those with monosomy for 17p, but reduced expression of the p53 gene is relatively common. We are currently trying to detect point mutations of the p53 gene by PCR technology especially in patients with monosomy for 17p.