Variations in DNA sequences flanking the insulin gene were studied in relation to noninsulin-dependent diabetes mellitus (NIDDM) in 87 unrelated Pima Indians at least 35 yr of age. DNA was isolated from nuclei of peripheral blood leukocytes and digested with restriction endonucleases. Less variation in this region was found in Pima Indians than in other racial groups previously studied. Only two classes of alleles (classes 1 and 3) were found, and there was virtually no variation within classes. At least one class 3 allele was found in 47% of the 38 nondiabetic subjects and in 37% of the 49 with NIDDM (odds ratio = 0.65, P = 0.4, 95% confidence interval for the odds ratio = 0.25 to 1.67). Homozygosity for class 3 alleles, however, was found only in diabetics. There were no differences according to genotype in obesity, fasting or postload glucose or insulin concentrations, or in the relationships between insulin and glucose concentrations. 61% (11/18) of the diabetics with a class 3 allele were receiving drug treatment for diabetes compared with only 26% (8/31) of diabetics without a class 3 allele (P = 0.03). The insulin gene polymorphism probably plays no important role in the genesis of NIDDM in Pima Indians, nor does it influence the glucose or insulin concentrations or their relationship to each other, but the class 3 allele, especially when homozygous in this population, may influence the severity of the disease as indicated by need for drug treatment.