Haplotypes of the insulin receptor gene were resolved in parents from Scandinavian nuclear families by studying the segregation of seven restriction fragment length polymorphisms (RFLPs). Of 97 unrelated parents, 41 had non-insulin-dependent diabetes mellitus (NIDDM). Considerable linkage disequilibrium in the region of the insulin receptor gene was found. Pairwise non-random associations were found between proximate RFLP sites, indicating the absence of recombinational hot spots between these sites. Thus, association studies between DNA polymorphisms at this locus and disease susceptibility genes could well be feasible in this population. Differences in the distribution of insulin receptor haplotypes were examined between NIDDM patients and healthy subjects. However, the differences observed were not statistically significant.