We evaluated the effects of gestational weight gain on neonatal birthweight women in whom gestational diabetes mellitus (GDM) was diagnosed after the 32nd week of gestation. The prevalence of macrosomia, the birthweight differences from 50th percentile value of a reference population, and the relationships among plasma glucose values during oral glucose tolerance test and neonatal birthweight were evaluated in 60 newborns from mothers with gestational diabetes mellitus divided according to pregravid body-mass index. Serving as controls were 132 newborns of mothers with normal glucose tolerance. The prevalence of macrosomia was higher in the GDM group; the neonatal birthweight difference above 50th percentile value was higher in newborns of mothers with GDM; and a strong relationship between maternal gestational weight gain and neonatal birth weight was present in all pregnant women. In conclusion, (1) the gestational weight gain is a good predictor of neonatal birth weight in all pregnant women; (2) GDM enhances the increase in neonatal size induced by excessive gestational weight gain alone, and (3) a weight gain of more than 9 kg makes the relative risk of macrosomia twofold higher in GDM than in control mothers.