Anorectal malformations (ARM) are major congenital malformations, which are assumed to have a multifactorial etiology with both genetic and environmental factors playing an important role.In a Dutch case-control study, we investigated the role of maternal and paternal risk factors in the etiology of ARM. Parents of 85 ARM cases and 650 controls filled in a questionnaire. Controls were children treated with ear ventilation tubes. The questionnaire contained questions about time to pregnancy (TTP), medical and family history, lifestyle factors, such as smoking, alcohol use, and vitamin intake, and occupation in the three months before and during pregnancy.Forty percent of ARM cases had also urogenital tract malformations. A higher occurrence of fever during the first trimester of pregnancy was found for case mothers compared to control mothers, resulting in an OR of 7.8 (95% CI:1.2-48.6). Maternal occupational exposure to industrial cleaning agents and solvents increased the risk of ARM five times (OR:5.3, 95%CI:1.5-19.4). Overweight (BMI ≥ 25 kg/m 2 ) before pregnancy also seemed to be associated with ARM (OR:1.9, 95%CI:1.1-3.3), as well as paternal occupational exposure to exhaust fumes (OR:2.0, 95%CI:1.0-4.0). Reported ARM in at least one first or second degree family member increased the risk of having a child with ARM more than thirty times (OR:33.9, 95%CI:2.9-398).This is the first study investigating a wide range of risk factors in the etiology of ARM. Potential risk factors may be found in fever during pregnancy, maternal overweight, and occupational exposures, but a familial component seems likely as well.