Prior research indicates risk for alcoholism is increased among individuals who begin to drink at an early age. We replicate and extend these findings, addressing causal and noncausal explanations for this association. Structured psychiatric interviews, including assessment of lifetime DSM-IV alcohol abuse and alcohol dependence (AD), were conducted with 8746 adult twins ascertained through a population-based twin registry. We found strong evidence for an association between early drinking onset and risk for AD, but less evidence for an association with alcohol abuse. The results of twin-pair analyses suggest that all of the association between early drinking and later AD is due to familial sources, which probably reflect both shared environmental and genetic factors. These results suggest the association between drinking onset and diagnosis is noncausal, and attempts to prevent the development of AD by delaying drinking onset are unlikely to be successful.