Using data on insurance premiums for policies with varying deductibles, together with a distribution of medical expenses, the authors estimate the relationship between deductibles and the demand for medical care. Estimates are limited to deductibles ranging from $50 to $1000. Results indicate that demand is quite sensitive to variation in a deductible in the region of $50 and becomes steadily less sensitive as the deductible rises above $75. This finding is consistent with a theoretical model of demand for medical care given a deductible, and with what is presently known about the responsiveness of demand to variation in coinsurance. Conclusions are that the size of the deductible will importantly affect the amount of public funds used in a national health insurance program and thus the distribution of payments among the population in a tax-financed program. No evidence was found that increasing the deductible increases expenditures by deterring efficacious preventive care, but the data are not well suited to test this hypothesis. 58 pp. Ref.