We evaluated the risk of Wilms' tumor in the offspring of women taking various medications during pregnancy in a case, control study conducted in Brazil. The study accrued 109 cases and 218 age- and gender-matched hospital controls. After adjustment for known confounders, we found a strong association with ingestion of dipyrone (odds ratio = 10.9; 95% confidence interval = 2.4-50) particularly in women from low-income families. Although dipyrone-containing analgesics are banned in Europe and North America, they are widely prescribed in Brazil and are given as free samples in neighborhood clinics providing free health care. The strong effect specific to low-income women may result from higher individual consumption compared with women at higher income levels.