A conventional and a computer search of the literature yielded 627 sequenced point mutations in the ras and p53 genes in 575 patients with leukaemia and myelodysplasia (MDS) out of a total of 4214 investigated. ras Mutations predominated in myeloid leukaemia and were more common in the disease in relapse than at presentation. There was no clinical, or haematological difference or difference in survival between ras positive and ras negative patients with acute myeloid leukaemia (AML) in adults or children, but ras mutations carried a poorer prognosis in childhood acute lymphocytic leukaemia and an increased risk of leukaemia in MDS. p53 mutations predominated in lymphoid leukaemia and were several fold more frequent in leukaemia in relapse than in the de novo disease, were associated with loss of the normal p53 allele (monosomy 17) in > 50% of cases and carried a poor prognosis in AML, MDS and chronic lymphatic leukaemia and a 3.8-fold increase risk of death in T cell acute lymphocytic leukaemia. There were 163 transitions for every 100 transversions, the expected number being ca 50. Consideration of the molecular mechanisms by which nitrous acid produces transitions allows transitions resulting from the deamination of cytosine to be distinguished from those resulting from the deamination of adenine. The former constitute 84.67% and the latter 15.33% of the 372 transitions present. Again purine-->pyrimidine and pyrimidine-->purine transversions form 80.35 and 19.65%, respectively, of the 228 transversions present. The possible bearing of this highly non-random distribution on the aetiology of point mutations in leukaemia and myelodysplasia is discussed.