The myelodysplastic syndrome (MDS) or preleukaemia is a haematological disorder characterized by low blood counts, bone marrow cells of abnormal appearance and progression to acute leukaemia in as many as 30% of patients. The distinctive preleukaemic and leukaemic phases of this disease make it an attractive model for neoplastic progression in human tumours. We reasoned that, because dominantly transforming genes (such as mutant alleles of ras proto-oncogenes) are found so frequently in acute leukaemia, the search for these genetic lesions during the clinical course of patients with MDS might give us insight into the function of oncogenes in leukaemogenesis. We report here that bone marrow cells from two of four patients with preleukaemia, and from one patient who progressed to acute leukaemia from MDS, contained a transforming allele of the Ki-ras proto-oncogene. In one preleukaemic patient, a novel mutation in codon 13 of this ras gene was detected in bone marrow cells harvested 1.5 years before the acute leukaemia developed. Our findings provide evidence that ras mutations may be involved in the early stages of human leukaemia.