To provide benchmarks for evaluating the costs of alternative proposals to provide insurance coverage for the uninsured, this study presents two sets of cost estimates derived from medical spending patterns of lower- or middle-income people with private insurance plans and those of people with public insurance coverage during 19961998. The analysis suggests that the uninsured would use $33.9$68.7 billion (in 2001 dollars) in additional medical care if they were fully insured. An increase in medical spending of this range would increase total health care spending by 36 percent and would raise health care's share of GDP by less than one percentage point. (Health Affairs Web Exclusive, June 4, 2003, see www.healthaffairs.org.)