We have studied point mutations in exons 5-8 of the p53 gene in the myelodysplastic syndromes (MDS) by using polymerase chain reaction (PCR) single-strand conformation polymorphism (SSCP) analysis and direct nucleotide sequencing. The subtypes examined were: refractory anaemia (RA), refractory anaemia with ring sideroblasts (RARS), chronic myelomonocytic leukaemia (CMML), refractory anaemia with excess blasts (RAEB), refractory anaemia with excess blasts in transformation (RAEBt), and acute myeloid leukaemia (AML) which had evolved from MDS. 26 cases of MDS were studied. 12 of these were sequentially sampled but none changed its p53 status during the time of the study (18 months). Four mutations (one nonsense and three missense) were identified. Each case with a mutation was of an advanced MDS subtype, suggesting that p53 mutation in these diseases is a terminal genetic event in the process of leukaemogenesis. The nonsense mutation inserted a premature stop codon in a case of AML which had evolved from RAEB; this mutation has been reported before in both chronic myeloid leukaemia (CML) and Burkitt's lymphoma. The three missense mutations have not previously been reported in haematological malignancies.