Little is known about causes of Wilms tumor. Because of the young age at diagnosis, several studies have looked at various birth characteristics. We conducted a registry-based case-control study involving 690 cases of Wilms tumor aged 0-14 years, occurring in Denmark, Finland, Norway or Sweden during 1985-2006, individually matched to five controls drawn randomly from the Nordic childhood population. Information on birth characteristics was obtained from the population-based medical birth registries. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) using conditional logistic regression analysis. We observed a distinct association between Wilms tumor and high birth weight (=4 kg) for girls (OR 1.97, CI 1.50-2.59) but not for boys (1.04, 0.78-1.38); overall, the OR was 1.43 (1.17-1.74). Among girls, risk increased by 28% (15-42%) per 500 g increase in birth weight. Large-for-gestational age girls also had a higher risk (2.48, 1.51-4.05), whereas no effect was seen for boys (1.12, 0.60-2.07). An association was seen with Apgar score at 5 min <7 for both sexes combined (5.13, 2.55-10.3). ORs close to unity were seen for parental age and birth order. In our large-scale, registry-based study, we confirmed earlier observations of an association between high birth weight and risk of Wilms tumor, but we found an effect only in girls. The higher risk of infants with low Apgar score might reflect hypoxia causing cell damage, adverse side effects of neonatal treatment or reverse causation as low Apgar score might indicate the presence of a tumor.