We estimate that at least 29 million Americans with private insurance are underinsured. That figure identifies the underinsured younger than 65 years by the risk of large out-of-pocket expenditures for an unusually expensive, catastrophic illness. A slightly smaller number, about 25 million, are underinsured by an alternate definition: they have insurance that pays a smaller proportion of claims than the plan with the largest enrollment in the federal employee program. The federal employee plan was the insurance standard proposed in several recent health system reform bills. Our estimate of the number of people who are underinsured for catastrophic illness is almost half again larger than the number that was widely cited during last year's debates on health system reform. That estimate was based on the same concept but was projected from a study published 10 years ago.