The goal of this study was to determine the factors that are related to utilization rates among older compared with younger enrollees in a health maintenance organization (HMO). Andersen's behavioral model was used as the conceptual framework for examining utilization rates. The approach was to determine the relative importance of predisposing, enabling, and medical need factors in explaining utilization rates among younger and older enrollees of an HMO. The study population included 2,603 adults enrolled in a large HMO. Three years of medical record data were linked with survey data for the analysis. The findings suggest that while the model as a whole explains about the same amount of variation in total doctor office visits for both age groups, the actual predictor variables are not the same. Predisposing factors are more important in explaining variation in utilization among the younger than the older enrollees. Enabling factors are more important in explaining variation in utilization among the older than the younger enrollees. Medical need factors explain about the same amount of variation in total doctor office visits for both age groups. Theoretic and practical implications are discussed.