Hazardous waste sites often contain substances harmful to fetal development. Using linked birth-hospital discharge and hazardous sites data for Washington State, we evaluated the association between malformation occurrence and maternal residential proximity to hazardous waste sites. Cases (N=63,006) were infants born 1987-2001 with malformations. Controls (N=315,030) were randomly selected infants without malformations born during these years. Distance between maternal residence and nearest hazardous waste site was measured using geographic information systems (GIS) software. Odds ratio (OR) estimates of the relative risk of malformation at varying distances were calculated. Relative to living >5 miles from a site, living < or = 5 miles was associated with increased risk of any malformations in offspring (for >2- < or = 5 miles: OR 1.15: 95% Confidence Interval (CI): 1.10, 1.21; for >1- < or = 2 miles: OR 1.26, 95% CI: 1.20, 1.32; for >0.5- < or = 1 miles: OR 1.28, 95% CI: 1.22, 1.35; for < or = 0.5 miles: OR 1.33, 95% CI: 1.27, 1.40.) Risk estimates varied by urban vs. rural maternal residence and by specific malformation type. Hazardous waste sites are often located within populated areas. Thus, the possibility of increased malformation occurrence among those in close proximity deserves closer scrutiny.