We analyzed gestational diabetes mellitus trends in New York City between 1990 and 2001 by using information obtained from birth certificates. Gestational diabetes diagnoses among women who delivered babies increased 46%, from 2.6% (95% confidence interval [CI]=2.5, 2.7) to 3.8% (95% CI=3.7, 3.9) of births. Prevalence was highest among South and Central Asian women (11%). Given risks for adverse fetal outcomes and maternal chronic diabetes, prompt screening is critical. Metabolic control should be maintained during pregnancy and assessed postpartum for women with gestational diabetes.