This study examines the effect of multiple prior years of health expenditures on the probability of enrollment in a Medicare HMO. Beneficiaries may require more than one year of prior expenditure data to form a reliable estimate of future expenditures if health expenditures have a significant transitory component. We used a logit model to estimate the influence of 1991-1993 Part A expenditures and demographic data on the choice of health plans in 1994. The results indicate that beneficiaries use multiple years of expenditures in their choice of health plan and the effect of prior spending declined with time.