Point mutations in the p53 tumor-suppressor gene are the most frequently identified genetic alterations in human malignancies. In order to evaluate the role of p53 mutations in the multistep process of leukemogenesis we studied 61 patients with myelodysplastic syndromes using single-strand conformation polymorphism analysis of polymerase chain reaction products as well as direct sequencing. Mutant alleles were observed in 1/14 refractory anemia with excess of blasts (RAEB) and 2/5 RAEB in transformation. The three mutations represented G:C to A:T transitions at codon 141 (exon 5) and codons 245 and 248 (exon 7), respectively. These data suggest that p53 mutations may contribute, albeit rarely, to the development of preleukemic disorders of the myeloid cell lineage.