Maternal fish consumption during pregnancy may affect children's asthma risk by modulating early-life immune development. Type of fish intake may be important because of differences in fatty acid content. To test this hypothesis, we conducted a nested case-control study, selecting subjects from the Children's Health Study, a population-based study of school-aged children in southern California. Cases had physician-diagnosed asthma and controls were asthma-free by age 5 years. Mothers or guardians provided information on fish consumption during pregnancy in telephone interviews. We computed odds ratio (OR) and 95% confidence interval (CI) by using conditional logistic regression models that accounted for the sampling. In children born to mothers with a history of asthma, the OR of asthma was 0.20 (95% CI = 0.06-0.65) when mothers ate oily fish at least monthly during pregnancy compared with no consumption (p(trend) = 0.006). Maternal oily fish consumption during pregnancy did not benefit children of non-asthmatic mothers. In contrast, fish stick (a source of trans-fats) consumption during pregnancy increased asthma risk in children (OR = 2.04; 95% CI = 1.18-3.51). Our results suggest that maternal oily fish intake during pregnancy may protect offspring from asthma; however, eating fish sticks during pregnancy may increase asthma risk in children.