Health care expenditures of 1,136 children whose families participated in a randomized trial, The Rand Health Insurance Experiment, are reported. Children whose families were assigned to receive 100% reimbursement for health costs spent one third more per capita than children whose families paid 95% of medical expenses up to a family maximum. Outpatient use decreased as cost-sharing rose for a variety of use measures: the probability of seeing a doctor, annual expenditures, number of visits per year, and numbers of outpatient treatment episodes. Hospital expenditures did not vary significantly among children insured with varying levels of cost-sharing. Episodes of treatment for preventive care were as responsive to cost-sharing as episodes for acute or chronic illness. The results give no reason not to insure preventive care as liberally as care for acute illness.