The effect of birth weight on white blood cell (WBC) count among blacks and whites was examined in 2,080 children (aged 4–11 years, 57.4% white, and 49.2% male), 892 adolescents (aged 12–17 years, 57.2% white, and 50.8% male), and 1,872 adults (aged 18–38 years, 68.4% white, and 41.9% male) from Bogalusa, Louisiana, in 2005. After adjustment for age, sex, race, body mass index, and smoking status (in adolescents and adults), the WBC count decreased across quartiles of increasing birth weight specific for race, sex, and gestational age in children (P_trend = 0.0007) and adults (P_trend = 0.005). In multivariate regression analyses that included the covariates above, birth weight was inversely associated with WBC count in children (β coefficients (unit, cells/μL per kg) = −256, −241, and −251 for whites, blacks, and the combined sample, with P = 0.003, 0.029, and <0.001, respectively) and in adults (β = −224 and −211 for whites and the combined sample, with P = 0.015 and 0.008, respectively). These results show that low birth weight is associated with increased systemic inflammation as depicted by the WBC count in childhood and adulthood, thereby potentially linking fetal growth retardation to cardiovascular disease and diabetes.