The impact of factors that influence diabetes mellitus (DM) and impaired glucose tolerance (IGT) incidence rates among former gestational diabetes mellitus (GDM) patients undermine attempts at interstudy comparisons. The recommended diagnostic standards for GDM by oral glucose tolerance test (OGTT) are the O'Sullivan and Mahan criteria and the World Health Organization (WHO) criteria for IGT, which result in prevalence rates of 2.5 and 7.2%, respectively, when applied to 752 unselected pregnant women. In applying the O'Sullivan and Mahan criteria, the current open-ended definition of GDM without rules either to exclude overt diabetes uncovered by pregnancy or to require a return to a normal OGTT after pregnancy is shown to be a major source of differences in subsequent incidence rates of diabetes. For subsequent nonpregnant diagnoses, the differences between WHO and National Diabetes Data Group criteria and the allowable modifications within each of the diagnostic standards all result in different incidence rates of diabetes. Review of 12 worldwide studies of diabetes among former GDM patients indicated a wide range of incidence rates, from 19 to 87% for combined DM and IGT and 6 to 62% for DM. In applying WHO DM criteria to GDM patients and control subjects, the excess risk of diabetes among GDM patients was 18% in Copenhagen and 30.9% in Boston, MA. The potential impact of varying observation periods within studies was seen when the application of an actuarial method added a further 50% to the Boston incidence rates of both GDM patients and control subjects. Although the variability in diabetes incidence rates is wide, there is broad general agreement on the predictive nature of gestational blood glucose levels.