Payment mechanism and patterns of use of medical services: the example of hypertension. This study explores the relationship between the use of medical services by hypertensive patients and mechanisms for payment within a single primary care practice. Three payment mechanisms were explored: public assistance, a capitated health maintenance organization (HMO), and fee-for-service. Patterns were examined across reimbursement type for the following variables: age, sex, visit reason, number of visits, medications, tests ordered, referrals made, and recommendations for follow-up visits. Illness severity was controlled in two ways: (1) by the study being focused on one diagnosis--mild to moderate hypertension, and (2) by concurrent chronic illnesses being enumerated and included in the analysis. Medical visits to the physician were examined over a 2-year period for 25 to 30 patients randomly sampled from each of the three payment mechanisms. Statistically significant differences were found for patient behaviors (total number of patient visits) and physician behaviors (number of medications and recommendations for revisits). The highest visit frequency was found for those on public assistance, followed closely by those covered by an HMO, and more distantly by those choosing fee-for-service. In a climate of cost consciousness, further study is needed to explore the impact of reimbursement mechanisms on the use of health care services. 