Factors influencing early formula supplementation in breast-fed neonates were examined among 726 women who were delivered of their first child in one of three metropolitan Washington, DC, hospitals. Thirty-seven percent of breast-fed neonates were given supplementary formula in the hospital. Mothers who gave birth at a university hospital were more likely to breast-feed exclusively (adjusted odds ratio 3.5; 95% confidence limit 2.1 to 5.9), after adjustment for maternal demographics, hospital factors (such as time of first breast-feed, demand feeding, delivery type, and rooming-in), and the maternal breast-feeding commitment. Aside from delivery hospital, a strong predictor of formula use was the time between birth and initiation of the first breast-feed. The longer a mother waited to initiate breast-feeding the more likely she was to use formula; the adjusted odds ratios for women who initiated breast-feeding 2 to 6 hours, 7 to 11 hours, and 12 or more hours postpartum were 1.1, 0.5, and 0.2, respectively. Feeding the baby on demand, having a vaginal delivery, deciding to breast-feed before pregnancy, having a college education, and being married also were moderately, though significantly, predictive of exclusive breast-feeding. The findings suggest that hospital influences can promote formula use and indirectly shorten breast-feeding duration, particularly those hospital practices that delay early initiation of breast-feeding.