The lateral distribution of external birth defects has not been reported in a comprehensive way, and patterns in this distribution have not been examined. This study presents the lateral distribution of 6,390 unilateral defects from among 102 defect categories in data collected by the Metropolitan Atlanta Congenital Defects Program. Among all defects, 49% (95% CI 48-51%) were right-sided. Among males and females, 51% (95% CI 50-53%) and 47% (95% CI 46-49%) of the defects, respectively, were right-sided. Of the 102 defect types, 57 had an excess of defects on the right side of the body; 39 had an excess of defects on the left side; and 6 were equally distributed. The excess on the right side was statistically significant for inguinal hernia, incarcerated inguinal hernia, microtia, preauricular sinus, talipes calcaneovalgus, and lambdoidal craniosynostosis. For the left side, the excess was statistically significant for preauricular tags, cleft lip, fused lip and cleft gum, cleft lip with cleft palate, congenital hip dysplasia, unstable hip, absent forearm or hand, anomaly of the knee, and skin tags. The percentage of right-sided defects among case subjects with unilateral defects was correlated with the percentage of males among all case subjects (r Ο 0.24, P Ο½ 0.05). Among male case subjects with unilateral defects, the correlation coefficient was 0.31 (P Ο½ 0.01), and among females with unilateral defects, it was 0.11 (P ΟΎ 0.10). Differences in the lateral distribution of specific birth defects may be due to subtle differences in morphogenesis on the left and right sides of the embryo brought about by establishment of left-right asymmetry prior to organogenesis. The fact that more defect categories were right-sided than left-sided may be related to the observation that mitochondrial maturation in rat embryos is delayed on the right side. The right side, therefore, may be more susceptible than the left to defects caused by prenatal hypoxia. The significant correlation between the percentage right-sided and percentage male may then also be related to the observation that male sex hormones lower the mitochondrial respiration rate in rats and increase rat sensitivity to chemical hypoxia. Investigators should consider reporting the laterality of specific defects in both laboratory and epidemiological studies of birth defects. Right-and left-sided defects should perhaps be considered separately in etiologic studies of birth defects.