We report the nucleotide (nt) substitutions of four unrelated glucose-6-phosphate dehydrogenase (G6PD)-deficient males. Only the mutation of G6PD Wayne was unique. It was a nt 769 C----G substitution causing a deduced substitution of glycine for arginine at amino acid 257. This mutation is in a region in which G6PD mutations have previously been associated with chronic hemolytic anemia. The mutation of G6PD Jammu and G6PD Viangchan were identical: a G----A mutation at nucleotide 871, predicting a Val----Met substitution at amino acid 291. However, these two variants differ with respect to the 1311 polymorphism, suggesting that they may have arisen independently. Enzyme from a child with chronic hemolytic anemia, designated G6PD 'LeJeune', proved to be due to a G----T substitution at nt 637, a change identical with that in 3 unrelated patients who had been reported previously as having G6PD Gastonia, Minnesota and Marion. These findings support the suggestion that both polymorphic and sporadic G6PD deficiency mutations in unrelated persons with G6PD deficiency are often the same, even when thought to be distinct on the basis of biochemical characterization.