We related perinatal outcome and quality of prenatal care between 1972 and 1980 to WIC benefits to pregnant women among 11,154,673 births in 1392 counties in 19 states and the District of Columbia, adjusted for time change and between-county variability. WIC was associated with increased first trimester prenatal care (4.1%, p less than 0.001), decreased inadequate prenatal care (-5.0%, p less than 0.001), longer duration of gestation (0.20 d, p less than 0.05), decreased preterm delivery (-0.92%, p less than 0.05) and increased birth weight (23 g, p less than 0.01). After autocorrelation was accounted for, significance levels for birth weight and frequency of preterm delivery were 0.05 less than p less than 0.10. Including counties with incomplete time series data, the estimated effect on birth weight was 23.9 g (p = 0.004) and on decreased fetal mortality was -2.3/1000 (p = 0.04). Among those with less than 12 y schooling, whites had a 1.8% reduction in preterm birth (p less than 0.05) and blacks a 2.0% reduction (p less than 0.05). The estimated effect on birth weight for less-educated whites was 46.6 g (p less than 0.001). The predominant effects of WIC were on improved physiologic status of the mother and fetus.