A representative sample of 817 American women aged 20–44 yr who were not pregnant and had no medical history of diabetes were given 75-g 2-h oral glucose tolerance tests (OGTTs). Although these conditions are somewhat different from those recommended for pregnant women (100 g glucose, 3-h OGTT), 3.8% of the women might have been considered to have met O'Sullivan and Mahan criteria for gestational diabetes mellitus (GDM) had they been pregnant. Prevalence was 2–3% below age 35 yr, similar to that found in studies of pregnant women, and rose to 8% at age 40–44 yr. Rates of women meeting World Health Organization criteria for gestational impaired glucose tolerance (G-IGT) rose steadily from 5% at age 20–24 yr to 11% at age 40–44 yr. Risk factors for non-insulin-dependent diabetes mellitus (NIDDM) including parental history and obesity were more prevalent among women meeting these criteria than among women in the entire group; the same risk factors are also more prevalent among pregnant women with GDM. The similarity of rates in this study to rates of GDM and G-IGT, together with their association with risk factors for NIDDM, indicate that these entities are compatible with undiagnosed glucose intolerance occurring before pregnancy and discovered during the metabolic testing that generally accompanies prenatal care rather than conditions that have an etiologic relationship to pregnancy.