The present study examined the degree to which alcohol use problems explained the relationship between cannabis use frequency and college functioning. Undergraduates (N = 546) mandated to an alcohol diversion program at a Midwestern United States university completed screening questionnaires between October 2003 and April 2006. Sobel's (1982) test of mediation showed that problematic drinking fully mediated cannabis use frequency's relationship to (a) lower grades and (b) greater legal problems. Findings underscore the notion that cannabis use's association with college functioning may depend on the severity of alcohol use problems. (Contains 2 tables.)