Little is known about ethnic differences in glucose tolerance during pregnancy. In this study we examined 3366 Hispanic, Chinese, black, and non-Hispanic white women in a universal screening program for gestational diabetes mellitus. After maternal age and body mass index were controlled, Chinese women had a significantly higher serum glucose level 1 hour after 50 gm of oral glucose (134.8 +/- 1.2, mean +/- SE) than any of the remaining three groups. Black women had a significantly lower value (113.3 +/- 1.3, mean +/- SE) than either Chinese or Hispanic women (124.4 +/- 0.9, mean +/- SE). Results for Hispanic women and non-Hispanic white women (121.4 +/- 1.6, mean +/- SE) were not different. The screening glucose levels of Chinese women were substantially higher than other ethnic groups even when women with gestational diabetes were removed from the analysis, indicating that the observed differences were not solely due to a higher frequency of gestational diabetes among the Chinese. The incidence of gestational diabetes was significantly greater for Chinese (7.3%) and Hispanic (4.2%) women than for black (1.7%) and non-Hispanic white (1.6%) women. Among women who had a 3-hour glucose tolerance test, the area under the glucose curve was significantly associated with maternal age and body mass index. The demonstrated heterogeneity of glucose tolerance between ethnic groups may be of importance in determining the threshold for diabetic fetopathy, and it is possible that ethnicity-specific standards will need to be developed.