Results indicated that college students who used both alcohol and marijuana were at a significantly higher risk for developing AOD problems than were students who only used alcohol, even when heavy drinking was taken into account. AOD problems included greater chances of legal problems, hangovers, damaging property, being in verbal arguments, and riding with an intoxicated driver. These findings remained after controlling for demographics and alcohol use behaviors. The analysis also revealed that dual users tended to be younger and to report consumption of a greater number of drinks per occasion and to report experiencing all AOD problems compared to alcohol-only users. Data were drawn from an ongoing evaluation of an environmental prevention program targeting alcohol-related problems among college populations. The evaluation employed a cross-sectional survey administered via telephone to 1,607 undergraduate students who were randomly selected from 2 large urban public universities between fall semester 2002 and spring semester 2003. The survey focused on substance abuse and AOD problems. Data were analyzed using the Statistical Package for the Social Sciences (SPSS) version 13.0 and included chi-square tests and multiple regression models. Future research should focus on both the underlying mechanisms contributing to substance-related problems for dual users and on whether alcohol-based interventions are effective in addressing the problems experienced by dual users. Tables, references