Background: Previous research has suggested that substantial comorbidity exists among childhood externalizing disorders, specifically attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD). Moreover, parent-child conflict predicts each of these disorders. Our goals were to determine whether parent-child conflict was associated with the comorbidity among ADHD, CD, and ODD, and to explicitly examine the etiology of this association via a genetically informative design. Methods: We compared the fit of the following 2 biometric models: the 2-factor common-pathway model, which examined genetic and environmental contributions to the relationship between conflict and the covariation among the 3 disorders, and the Cholesky model, which examined the relationship between conflict and each disorder individually. The sample consisted of 808 same-sex 11-year-old twin pairs from the Minnesota Twin Family Study, a population-based sample of Minnesota twins and their families. Main outcome measures included symptom counts for ADHD, CD, and ODD, obtained from structured interviews administered to twins and their mothers. Parent-child conflict was assessed via mother and twin reports of the Parental Environment Questionnaire. Results: The 2-factor model provided a better fit to the data. These results indicated that conflict accounted for 33% of the covariation among the disorders, via genetic and environmental factors. Conclusions: Parent-child conflict appears to act as a common vulnerability that increases risk for multiple childhood disorders. Furthermore, this association is mediated via common genetic and environmental factors. These findings support the idea that the comorbidity among these disorders partially reflects core psychopathological processes in the family environment that link putatively separate psychiatric disorders.