Surprisingly little is known about long-term spending patterns in the under-65 population. Such information could inform efforts to improve coverage and control costs. Using the MarketScan claims database, we characterize the persistence of health care spending in the privately insured, under-65 population. Over a 6-year period, 69.8% of enrollees never had annual spending in the top 10% of the distribution and the bottom 50% of spenders accounted for less than 10% of spending. Those in the top 10% in 2003 were almost as likely (34.4%) to be in the top 10% five years later as one year later (43.4%). Many comorbid conditions retained much of their predictive power even 5 years later. The persistence at both ends of the spending distribution indicates the potential for adverse selection and cream skimming and supports the use of disease management, particularly for those with the conditions that remained strong predictors of high spending throughout the follow-up period.