This case-control study, conducted in a California county that had a local incident of water contamination in 1981, investigated the relation between a mother's reported consumption of tap water during pregnancy and congenital cardiac anomalies in their offspring born during 1981-1983. Data were obtained from telephone interviews with 145 mothers of children born with a severe cardiac anomaly and 176 mothers of children born without such an anomaly. A positive association between a mother's consumption of home tap water during the first trimester of pregnancy and cardiac anomalies in her infant was unrelated to the incident of water contamination, the mother's race, or her educational level. A negative relation was found between a mother's use of bottled water and cardiac anomalies among the infants. These findings corresponded primarily to births in 1981. These data could not fully distinguish between a potential causal agent in the water and differential reporting of exposure by study subjects.