The effect of paternal age on the risk of birth defects among offspring is less well studied than the effect of maternal age, with few comprehensive epidemiologic studies having been conducted. Advanced paternal age has been shown to be associated with an increase in new dominant mutations that result in particular congenital anomalies. The relationship between paternal age and more common birth defects, for example, cardiac defects, has not been as extensively evaluated. Therefore, a total of 4,110 cases of congenital heart defects was identified from the British Columbia Health Surveillance Registry. Matched controls were obtained from the birth files of British Columbia for the years 1952-1973. Prevalence odds ratios for paternal age, adjusted for maternal age and other factors, were estimated for 8 cardiac defect groups. A suggestive general pattern of increasing risk with increasing age among cases (excluding chromosomal anomalies) relative to controls was found for ventricular septal defects (VSD), atrial septal defects (ASD), and patent ductus arteriosus (PDA). In addition, an increased risk among men younger than 20 yr was found for VSD and ASD. These findings are consistent with the results of some previous epidemiologic studies. Based on the results of this study it is estimated that for cardiac defects such as VSD, approximately 5% of cases may be due to advanced paternal age (> 35 yr), possibly through dominant mutations.