Objective: To identify determinants of low birth weight (LBW) in Karachi, Pakistan, including environmental exposures and nutritional status of the mother during pregnancy. Design: Cross-sectional study. Participants: Five hundred and forty mother-infant pairs. We interviewed mothers about obstetric history, diet and exposure to Pb. We measured birth weight and blood lead level (BLL). We performed multiple log binomial regression analysis to identify factors related to LBW.Results: Of 540 infants, 100 (18.5%) weighed 208.7 mg/d), infants of mothers with MUAC less than or equal to the median and dietary vitamin C intake \u3e 208-7 mg/d (adjPR = 10.80, 95 % CI 1.46, 79.76), mothers with MUAC above the median and vitamin C intak