The prevalence at birth of gastroschisis, a rare abnormality of the abdominal wall, appears to have increased over the past decade. To characterize risk factors that might explain this increase, birth certificates for Washington State residents were compared for 62 infants born with gastroschisis during the years 1984 to 1987 and 617 randomly selected unaffected infants matched for birth year. After simultaneously adjusting for 14 potential risk factors, 4 factors stood out. Infants born during January, February, or March were at greater risk than infants born in any other months (odds ratio 2.2, 95% confidence interval 1.1, 4.1). Mothers less than 25 years old were at greater risk than mothers 25 years and older, with the highest risk to mothers less than 20 years old (odds ratio 4.1, 95% confidence interval 1.4, 12.0). Women who smoked during pregnancy were at greater risk than women who did not smoke (odds ratio 2.0, 95% confidence interval 1.03, 3.8). Finally, mothers receiving inadequate prenatal care were at greater risk than mothers receiving adequate prenatal care (odds ratio 2.1, 95% confidence interval 0.99, 4.6). Unidentified behavioral and environmental exposures may explain the associations with month of birth, maternal age, and prenatal care. However, smoking during pregnancy is a plausible risk factor that should be examined further as an explanation of the apparently increasing prevalence at birth of gastroschisis in developed nations.