The authors conducted an exploratory case-control study of paternal employment as a fire fighter and the risk of birth defects among offspring. By means of a population-based registry in British Columbia, 22,192 liveborn children with birth defects were identified for the period, 1952-1973. Two comparisons were made to evaluate the risk among offspring of firemen: 1) comparison with the risk among offspring of fathers employed in other occupations using a matched (two per case) control group, and 2) comparison with the risk among offspring of policemen. Among the 20 birth defect groups studied, an association was found for paternal employment as a fire fighter (relative to all other occupations) and increased risks were observed for ventricular septal defects (odds ratio (OR) = 2.70, 95% confidence interval (Cl) 1.02-7.18) and atrial septal defects (OR = 5.91, 95% Cl 1.60-21.83) among offspring. When compared with policemen, firemen had increased risks of having a child with a ventricular septal defect (OR = 4.68, 95% Cl 1.66-13.17) and atrial septal defect (OR = 3.76, 95% Cl 1.40-10.07). Analyses using a pathogenetic classification for these heart defects yielded similar increased odds ratios for firemen.