The study examines the medical care (hospital, physician, drug, diagnostic) utilization and expenditures of low income persons enrolled in a prepaid health plan with a matched group of Medicaid recipients. The study also examines the medical care utilization of low income persons enrolled in a prepaid health plan with a similar group of low income persons enrolled in the health plan but also eligible for Medicaid benefits. Utilization and population-at-risk data were obtained from the Kaiser-Permanente Medical Care Program of Portland, Oregon and from the State of Oregon Welfare Division. A hypothesis of lower hospital utilization by low income enrollees compared with Medicaid recipients was accepted. A hypothesis of higher ambulatory care utilization was accepted for diagnostic procedures and prescription use, but rejected for office visit utilization. An analysis of the findings appeared to implicate the Medicaid program for differences observed. The hypotheses of no significant differences in inpatient and ambulatory medical care utilization of low income health plan enrollees with and without Medicaid eligibility were generally rejected. Low income enrollees with concurrent Medicaid had consistently higher utilization rates for all services resulting in substantially higher medical care expenditures per person. The findings appear to contribute some useful information to planning or establishing policy for Medicaid Prepayment programs or other programs enrolling low income persons in prepaid health plans or HMOs.