To reduce Medicare costs, Medicare beneficiaries are being encouraged to enroll in “risk contract” HMOs. This paper explores the financial consequences to the elderly of joining a Medicare risk HMO. Using a new method for estimating consumer financial vulnerability called the illness episode approach, we modeled the out-of-pocket costs associated with thirteen illnesses of varying severity for beneficiaries with traditional Medicare coverage only and for beneficiaries who join one of two Los Angeles HMOs which charge no additional premium. The typical total charges for a year's treatment of these thirteen illnesses in Los Angeles in 1986 ranged from a low of $856 for moderate hypertension to a high of $28,411 for care of a severe stroke. For beneficiaries with traditional Medicare whose providers did not accept assignment, out-of-pocket costs ranged from $539 to $14,676 and from a low of 7.7 percent to a high of 84.1 percent of total charges. Out-of-pocket costs are considerably reduced in the two Medicare HMOs in this high-cost market; beneficiaries had modeled out-of-pocket costs ranging from $11 to $7,478 and from less than 0.1 percent of total charges to 60 percent of charges. Reductions in financial vulnerability ranged from over 20 percent to 99.3 percent. The relation of these reductions to altered benefit structures and the policy implications of the results are discussed.