Since 1979, the CDC Pregnancy Nutrition Surveillance System (PNSS) has monitored behavior and nutritional risk factors among low-income pregnant women participating in public health programs. Although the states contributing to the system have varied over the period, the PNSS is able to characterize the behavior and health outcomes of pregnant women from diverse low-income populations. In 1990, 66.2% of the women in the system initiated prenatal care during the first trimester of pregnancy; 26.4% smoked during pregnancy. Since 1979, the prevalence of smoking remained relatively stable for white women, but declined for blacks and Hispanics. Prepregnancy body mass index (BMI, defined as kg/m2) showed marked changes from 1979 through 1990; the prevalence of underweight (BMI < 19.8) declined steadily and the prevalence of overweight (BMI > 26) increased steadily. In 1990, 39.3% of the women had gestational weight gains below levels recommended by the National Academy of Sciences. Both prepregnancy underweight and inadequate gestational weight gain were associated with greater risk for low birth weight in the PNSS. The prevalence of anemia at each trimester has remained stable since 1979. In 1990, 9.8%, 13.8%, and 33% of the women reported by the PNSS were anemic in the first, second, and third trimesters, respectively. Anemia in the first trimester appeared to be strongly associated with a high risk of low birth weight; this association was attenuated in later trimesters. These findings indicate the need to improve iron nutrition among low-income women.