The objectives were to characterize and determine the predictors of birth weight among women with gestational diabetes mellitus (GDM), and to establish whether these predictors vary by prepregnancy body mass index (BMI). A cohort of 353 women with GDM was examined by using data from clinic charts and the McGill Obstetrical and Neonatal Database. Dietary treatment decreased the rate of weight gain (from 0.35 to 0.16 kg/wk, P < 0.0001) and fasting serum glucose (from 4.8 to 4.2 mmol/L, P < 0.001). Positive predictors of infant birth weight among the underweight and normal-weight women (BMI < or = 26) included prepregnancy BMI, height, parity, prediagnostic rate of weight gain, postprandial serum glucose, and gestational duration. Among the overweight and obese women (BMI > 26) the only variables that predicted increased birth weight were prediagnostic rate of weight gain, and fasting or postprandial serum glucose. Hence, prepregnancy body mass index exerts a strong effect on predictors of infant birth weight in GDM, indicating the need to consider prepregnancy weight in the treatment approach.