The Private Payers Study analyzed the health care utilization and expenditures of people with potentially disabling chronic conditions in private health plans. We chose two alternative methods to define this group. Both used medical claims data to identify utilization and illness. One method relied on the clinical judgment of health care experts to identify diagnosis codes and utilization patterns that indicate probable future disability. The second method drew on a national survey to determine diagnosis codes corresponding to ability-limiting conditions--that is, those conditions often accompanied by limitations in activities of daily living. By comparing results from these two sources, the Private Payers Study presented a reliable picture of the experiences of people with potentially disabling chronic illness and illustrated how the choice of definition altered the estimated prevalence in the privately insured populations we studied.