Because marijuana use often precedes the use of other psychoactive substances, it has been characterized as a gateway to these other substances. The present study used data from both monozygotic (MZ) and dizygotic (DZ) twin pairs drawn from the National Longitudinal Study of Adolescent Youth (Add Health) to examine the "gateway effect" role of earlier marijuana on later hard drug use. Difference score analyses reveal that within-pair differences in earlier marijuana use, controlling for differences in earlier hard drug use, and peer marijuana use predicted later within-pair hard drug use differences for DZ twin pairs. In contrast, earlier differences in marijuana use among MZ twin pairs did not predict later hard drug use differences. Rather than supporting the interpretation that earlier marijuana use "triggers" later hard drug use, these results suggest that the longitudinal pattern of drug use that has been interpreted as the "gateway effect" might be better conceptualized as a genetically influenced developmental trajectory.