This study tests the premise of peer cluster theory as it applies to individual alcohol use, and makes a comparative analysis between its ability to explain alcohol use and marijuana use. Using the results of a 1996 drug and alcohol survey of 1312 Western Kentucky University students, path analysis was used to measure the influence of six of peer cluster theory's psychosocial characteristics on the percentage of the respondent's college friends who use alcohol. All of these variables were then regressed on the respondent's alcohol use. The results of the causal models did show some support for peer cluster theory. The direct effect of the student's association with alcohol-using peers on individual alcohol use was shown to have the strongest direct influence on this outcome variable. However, a few limitations of this theoretical perspective were identified. The causal model for alcohol use showed that the indirect influence of two of these psychosocial characteristics (parental attitudes on alcohol use and success in school) was weaker than their direct influence on individual alcohol use. And, the comparative analysis showed that peer cluster theory is better suited to explain the use of marijuana than the use of alcohol.