The effect of maternal weight gain on birth weight in 2946 live births with delivery after 37 weeks' gestation was studied at Moffitt Hospital, University of California (San Francisco), between September, 1980, and December, 1983. The sample was stratified into four categories according to prepregnancy weight for height with use of a body mass index. To study the effect of maternal weight gain on infant birth weight, multiple regression analysis, controlled for selected covariables, was carried out on the entire sample and on each prepregnancy weight group. For the entire sample, both pregravid body mass and weight gain significantly influenced birth weight. For the underweight, ideal weight, and moderately overweight women, each kilogram of maternal weight gain significantly increased birth weight. This study supports recent evidence for the association between maternal weight gain and birth weight, but only for women whose prepregnancy weights are 135% of ideal or less. These results suggest that recommendations for a minimum weight gain for obese women are unnecessary.