OBJECTIVE - To determine the frequency of screening for gestational diabetes mellitus (GDM) among a population receiving regular prenatal care and to assess the extent to which National Diabetes Data Group (NDDG) criteria for the diagnosis of GDM are used by practicing obstetricians. RESEARCH DESIGN AND METHODS - We studied participants in the Nurses' Health Study II, a large prospective cohort study of 116,678 nurses aged 25-42 years in 1989. A total of 422 women who reported a first diagnosis of GDM between 1989 and 1991 were sent supplementary questionnaires regarding diagnosis and treatment, and medical records were requested for a subset of 120 to validate self-reported GDM and assess criteria used for diagnosis. A sample of 100 women who reported a pregnancy not complicated by GDM were sent questionnaires addressing GDM screening and prenatal care. RESULTS - Among a sample of 93 women who reported a pregnancy not complicated by GDM and responded to the supplementary questionnaire, 16 (17%) reported no glucose loading test: 69% of unscreened women had one or more risk factors for GDM. Among a sample of 114 women who self-reported GDM in a singleton pregnancy and whose medical records were available for review, a physician diagnosis of GDM was confirmed in 107 (94%). Records and supplementary questionnaires indicated that oral glucose tolerance tests (OGTTs) were performed in 96 (86%) of these women. Of women with a physician diagnosis of GDM whose OGTT results were available, 25% failed to meet NDDG criteria for this diagnosis, although all had evidence of abnormal glucose homeostasis. CONCLUSIONS - Screening for GDM is not universal, even among a group of health professionals in whom screening prevalence is likely to be higher than in the general population. Diagnostic criteria for GDM among obstetricians in practice remain nonstandard despite NDDG recommendations. Better understanding of the implications of different degrees of glucose intolerance and of varying GDM screening and management strategies is required to make policy recommendations for appropriate and cost-effective care.