We evaluated the effects of gender and family history for alcoholism on the co-occurrence of anxiety disorders and alcohol disorders ("comorbidity") in a large university student sample (N = 489). A structured diagnostic interview was used to elicit life-time histories of anxiety disorder (29%) and alcohol disorder (26%). The proportion of subjects receiving an alcohol diagnosis was significantly greater among those with an anxiety diagnosis (39% vs. 21%). Male subjects, as well as those with a parental history of alcoholism, had a significantly greater base-rate of alcohol disorder than did other subjects. However, these factors did not affect the relative increase in risk for an alcohol disorder associated with the presence of an anxiety disorder; that is, these factors did not interact with (i.e., moderate) comorbidity risk. In discussing these results, we emphasize issues relevant to identifying and distinguishing between interaction effects and base-rate effects in comorbidity studies.