Wilms tumor (WT) is a childhood kidney cancer with the highest rate of occurrence before the age of 2. Since it is rare, previous research has been limited and few risk factors have been established. We used a case-cohort design to examine the influence of birth characteristics on occurrence of WT in Minnesota. A total of 2,188 cases of cancer diagnosed in children aged 28 days to 14 years from 1988 to 2004 were identified using the Minnesota Cancer Surveillance System (MCSS). For each case, 4 children were randomly selected from Minnesota birth records during 1976–2004, frequency matched on birth year. Thus, a total of 8,752 children comprised the subcohort for the study, who in this analysis, served as comparison to the 138 cases of WT. Study variables included parental demographics, maternal pregnancy history and conditions and health and conditions of the child at birth. Associations with WT were assessed using hazard ratios (HR) and 95% confidence intervals (CI) calculated from stratified Cox regression models. We found an increased risk of WT for children who were large for gestational age compared to those average for gestational age and for children with congenital abnormalities. There was also an increased risk for children with a birth weight > 4,000 g compared to those with a birth weight between 2,500 and 4,000 g. All other factors examined showed no association with WT. This study contributes to the mounting evidence that children with large size at birth have an increased risk of WT. © 2007 Wiley-Liss, Inc.