Diabetes in pregnant Mexican-American women is a serious and expensive health problem. At the University of California, San Diego Medical Center, 44% of pregnant women are Mexican American. In the Diabetes in Pregnancy Clinic, only 7% of women with insulin-dependent diabetes are in this ethnic group compared with 66% of non-insulin-dependent diabetic patients and 51% of those with gestational diabetes mellitus (GDM). GDM is the most common complication of pregnancy in Mexican Americans with a prevalence approximately three times higher than that of whites (4.5 vs. 1.5%). Mexican-American obese GDM subjects had more frequent cesarean sections and were more likely to have complications of premature rupture of membranes and preterm labor (NS). Polycythemia and sepsis also occurred more often in their infants. Anthropometric measurements in infants of both lean and obese GDM subjects differed from those of infants of mothers without GDM. Infants of lean mothers with GDM were heavier and longer than those of lean mothers without GDM. In addition, they had increased waist-hip ratio and triceps and subscapular skin folds. Infants of obese mothers with GDM were heavier than those of lean mothers with GDM. Moreover, they were longer (P < 0.04); had a higher body mass index (P < 0.04); and larger waist and hip circumferences (P < 0.03) and buccal (P < 0.01), subscapular (P < 0.01), and sum of skin-fold measurements (P < 0.03). Our observations indicate that pregnant diabetic Mexican-American women have predominantly GDM and non-insulin-dependent diabetes. They represent a major public health problem because of increased maternal and neonatal morbidity.