OBJECTIVES: This research examines the influence of different combinations of health insurance and types of payments on physicians'prescribing patterns in the case of two major chronic conditions: hypertension and diabetes in the USA. METHODS: The study was based on information collected in physicians' practices, obtained from medical records and physician visits. The analysis used simple and ordinal logistic regressions. Special attention was given to Medicare patients due to the lack of coverage for prescription drugs in this program. Two samples of individuals were extracted from the 1996 US National Ambulatory Survey, 1of 1844 individuals diagnosed with hypertension and 1 of 694 individuals diagnosed with diabetes. RESULTS: There was a significant reduction in likelihood of access to drug therapy for patients with Medicare only compared to those patients with other types of insurance. Patients with Medicare only that were in a Health Maintenance Organization or had a prepaid type of payment were more likely to get prescribed drug therapy than patients with Medicare cover only and whom had other forms of payment such as fee for service or payment with a preferred prescriber organization. However, strong differences on the impact of different insurance plans exist between the two conditions. CONCLUSIONS: Results on both samples suggest that patients' health insurance status does have some influence on physician prescribing decisions, especially for patients covered with Medicare. This research provides evidence at the stage of a physician 's visit, that the lack of additional coverage for prescription drugs may limit access to both prescribed drug therapies during the visit.