We studied 738 live singleton births in Papua New Guinea to examine the contribution of ethnic origin and environmental factors to birth weight and gestation. Maternal history, examination and post-partum anthropometry, infant anthropometry and clinical gestational assessment were performed, and placental blood slides and histology were examined. Mothers from Highlands provinces who delivered on the coast had larger babies (mean birthweight 3.34 SD 0.48 kg) than either indigenous coastal mothers (mean 3.10 SD 0.51 kg) or those migrating to Port Moresby from other coastal regions (mean 3.14 SD 0.51 kg). This was due to a combination of longer gestation and better intrauterine growth (assessed by birthweight standard deviation scores). Gestation was slightly longer in Highlands mothers delivering on the coast compared with those delivering in the Highlands, but birthweight standard deviation scores were unchanged. Malarial infection of the placenta was almost never found. Stepwise regression analysis showed that parity, maternal height, Highlands origin and maternal body mass index (weight/height2) were all significant predictors of birthweight standard deviation score, while maternal body mass index and Highlands origin were predictors of gestational length. We conclude that Highland mothers have a tendency to deliver heavier babies, and do not demonstrate the fetal growth-retarding effects of altitude seen in other races. Although the incidence of low birthweight was only 7.1% in our study, we found that indices of maternal nutrition were predictors of birth weight.