A sample of 1,042 adults was drawn from an employed population who were given the option of enrollment in any of three prepaid, HMO-type organizations, one of which was an open-panel foundation, or continued subscription in a conventional Blue Cross/Blue Shield plan. The subjects in the sample were interviewed to examine three hypothesized relationships: (I) Enrollees in any HMO will exhibit higher levels of health concern than members of other health plans. (2) Users of preventive services will exhibit a higher level of health concern than nonusers. (3) Expressed health concern is a predictor of the rate of preventive use. Data were analyzed by bivariate and multivariate techniques. Support was found for each hypothesis. We found that, although the level of perceived health status did not vary across enrollees in the various plans, those who chose a closed-panel plan reported the highest level of health concern. Health concern was positively related to the use of preventive services, and the mean number of visits as well as the percentage of individuals who use them were the largest for the members of the closed-panel plans. We also found that health concern was a predictor of preventive use. Regression analysis was used to explore various relationships between health status, health concern, plan membership, and the use of preventive services. The hypothesis of adverse selfselection is questioned. The generalizability of the findings is limited to employed, essentially middle-income groups.To change the orientation of the medical system from one of crisis medicine to preventive care requires not only an appropriate restructuring of the delivery organizations but compatible client attitudes and behavior as well. An advantage claimed for certain organizational configurations that are generally known as Health Maintenance Organizations (HMOs) is their relative emphasis on preventive care? Their ability to achieve that objective is dependent in part on the attitudes and behaviors of their subscribers.The issue examined here is whether those who choose to enroll in an HMO express more concern with maintaining their health and engage in more preventive care behavior than do those who choose not to enroll. Further, the issue of whether more concern for health and preventive