OBJECTIVES: The associations of infant birth outcomes with maternal pregravid obesity, gestational weight gain, and prenatal cigarette smoking were examined. METHODS: A retrospective analysis of 1343 obese and normal-weight gravidas evaluated the associations of cigarette smoking, gestational weight change, and pregravid body mass index with birthweight, low birthweight, and small- and large-for-gestational-age births. RESULTS: Smoking was associated with the delivery of lower-birthweight infants for both obese and normal-weight women, and gestational weight gain did not eliminate the birthweight-lowering effects of smoking. Women at highest risk of delivering lower-birthweight infants were obese smokers whose gestational gains were less than 7 kg and normal-weight smokers whose gestational gains were less than 11.5 kg. CONCLUSIONS: To balance the risks of small and large-size infants, gains of 7 to 11.5 kg for obese women and 11.5 to 16 kg for normal-weight women appear appropriate.