BACKGROUNDAnorectal malformations ARM are major congenital malformations that usually require a multitude of surgical procedures at a very early age and have a large impact on the lives of patients and their parents. The causes of ARM are still largely unknown, but they are assumed to have a multifactorial etiology. A few studies focused on environmental risk factors, but evidence is still scarce.METHODSIn this Dutch casecontrol study 1996–2008, 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.RESULTSA higher occurrence of fever during the first trimester of pregnancy was found for case mothers compared to control mothers odds ratio OR, 5.1; 95 Confidence Interval CI, 0.9, 28.1. Maternal occupational exposure to industrial cleaning agents and solvents increased the risk of ARM three times OR, 2.9; 95 CI, 0.9, 9.3. Overweight Body Mass Index BMI ≥ 25 kgm2 before pregnancy also seemed to be associated with ARM OR, 1.8; 95 CI, 1.1, 2.8, as well as maternal multivitamin use during pregnancy OR, 1.6; 95 CI, 1.0, 2.7, paternal smoking OR, 1.8; 95 CI, 1.1, 2.9, and paternal occupational exposure to exhaust fumes OR, 1.9; 95 CI, 1.0, 3.6. Reported ARM in at least one first or seconddegree family member greatly increased the risk of having a child with an ARM OR, 40.3; 95 CI, 4.8, 342.8.CONCLUSIONSThis study revealed potential risk factors for ARM, including fever during pregnancy, maternal overweight, use of multivitamins, paternal smoking, and occupational exposures, but a familial component seems important as well. Birth Defects Research Part A, 2010. © 2010 WileyLiss, Inc.