A DNA sequence polymorphism, revealed by digestion of genomic DNA with the endonuclease Xba1 and hybridisation with a complementary DNA clone for a human glucose transporter, yields two alleles (sizes 6·2 kbp, the X1 allele; or 5·9 kbp, the X2 allele). The genotype frequencies were investigated in three non-insulin-dependent diabetic populations. The frequencies (%) of X1.X1, X1.X2, and X2.X2 were 13, 51, and 36 among 89 North European diabetic subjects, and 8, 38, 54 among their 104 controls (χ² test p<0·02; G-test p<0·02). For 53 South European diabetic patients the frequencies were 19, 50, 31, and for their 41 controls they were 2, 58, 40 (χ² test p <0·02, G-test p <0·01). The corresponding figures were 6, 55, 39 for 45 Japanese patients and 0, 28, 72 for a further 49 controls (χ² test p<0·01; G-test p<0·001). The occurrence of the association of the X1 allele with diabetes in three separate populations suggests that the polymorphic site may be close to a diabetogenic locus on chromosome 1.