Restriction fragment length polymorphisms in the insulin gene hypervariable region are compared among 93 women with gestational onset diabetes mellitus and 146 women with normal glucose tolerance during pregnancy. No significant differences in gene or genotype frequencies were observed in the overall sample (p greater than 0.50). However, an increased frequency of one allele (class 1) was observed among nonoverweight patients with gestational onset diabetes mellitus with elevated fasting plasma glucose levels compared with age-, race-, and parity-matched control subjects (p = 0.061). These data suggest that gestational onset diabetes mellitus is a heterogeneous disorder with respect to both genotypic and phenotypic characteristics, and that restriction fragment length polymorphisms near the insulin gene may serve as a molecular marker for susceptibility to gestational onset diabetes mellitus only in some women.