A study was conducted on a cohort of 476 women (364 black, 112 white), who attended the Jefferson County Health Department clinic for their prenatal care, to ascertain the relationship between maternal serum zinc concentration measured early in pregnancy and birth weight. For all subjects maternal serum zinc was significantly related to birth weight after various independent determinants of birth weight were controlled for. The data in this study indicate a threshold for maternal serum zinc concentration below which the prevalence of low birth weight increases significantly. Pregnant women who had serum zinc concentrations in the lowest quartile had significantly higher prevalence of low birth weight than did those mothers who had serum zinc concentrations in the upper three quartiles during pregnancy. These findings suggest that maternal serum zinc concentration measured early in pregnancy could be used to identify those women at higher risk of giving birth to a low-birth-weight infant.