The myelodysplastic syndromes (MDS) have a significant frequency of evolution into acute myeloid leukaemia (AML). Approximately 30% of MDS patients show activating mutations of the N-RAS proto-oncogene, and these patients are at increased risk of leukaemic evolution. Long-term survivors of aplastic anaemia (AA) and paroxysmal nocturnal haemoglobinurea (PNH) are also at significant risk of developing AML. We have screened peripheral blood DNA from 42 AA patients and 15 PNH patients for the presence of N-RAS point mutations. No mutations were detected in these samples, indicating that the mechanisms of evolution into AML may be different from those in MDS.