A retrospective analysis of 4,094 deliveries among Navajo Indian women was carried out to determine the prevalence of gestational diabetes mellitus and diabetes antedating pregnancy. Three data sources--a local prenatal registry, a delivery room log, and hospital discharge records--were evaluated for their usefulness as surveillance systems for gestational diabetes. In all, 177 cases of gestational diabetes and 13 cases of preexisting diabetes were identified, giving a prevalence of maternal diabetes in pregnancy of 4.6%. When women with preexisting diabetes or documented gestational diabetes during a previous pregnancy were excluded, the prevalence of gestational diabetes during the study period was 3.4%. Although each data source used separately failed to identify 20% to 40% of diabetic pregnancies, more than 97% of cases were identified using a combination of the prenatal registry and the delivery log.