Using data from two population-based case-control studies, we investigated whether maternal residential proximity to hazardous waste sites increased the risk for neural tube defects, conotruncal heart defects, and oral cleft defects in California. We obtained a residential history by interview for mothers of 507 neural tube defect cases (82.7% of eligible) and their 517 controls (84.6%); and 201 heart cases (84.4%), 439 cleft cases (82.2%), and their 455 controls (72.1%). We identified the locations of 764 inactive hazardous waste sites and systematically collected information on site-related contamination for the subset of 105 National Priority List sites. After controlling for several potential confounders, we found little or no increased risk for maternal residence in a census tract containing a site [odds ratio (OR) = 0.9, 95% confidence interval (CI) = 0.7-1.3 for neural tube defects; OR = 1.3, 95% CI = 0.8-2.1 for heart cases; OR = 1.2, 95% CI = 0.8-1.8 for clefts], but elevated risks for neural tube defects (OR = 2.1, 95% CI = 0.6-7.6) and heart defects (OR = 4.2, 95% CI = 0.7-26.5) for maternal residence within 1/4 mile of a National Priority List site. Furthermore, we observed elevated ORs (> or = 2.0) for neural tube defects and heart defects in association with maternal residence within 1 mile of National Priority List sites containing selected chemical contaminants. Among controls, only 0.6% and 4.4% lived within 1/4 mile and 1 mile of a National Priority List site, respectively, resulting in imprecision in risk estimation.