OBJECTIVE: To describe the rate of increase of the population of adults seeking care as inpatients in children's hospitals over time. PATIENTS AND METHODS: We analyzed data from January 1, 1999, to December 31, 2008, from patients hospitalized at 30 academic children's hospitals, including growth rates according to age group (pediatric: aged <18 years; transitional: aged 18-21 years; or adult: aged >21 years) and disease. RESULTS: There were 3 343 194 hospital discharges for 2 143 696 patients. Transitional patients represented 2.0%, and adults represented 0.8%, totaling 59 974 patients older than 18 years. The number of unique patients, admissions, patient-days, and charges increased in all age groups over the study period and are projected to continue to increase. Resource use was disproportionately higher in the older ages. The growth of transitional patients exceeded that of others, with 6.9% average annual increase in discharges, 7.6% in patient-days, and 15% in charges. Chronic conditions occurred in 87% of adults compared with 48% of pediatric patients. Compared with pediatric patients, the rates of increase of inpatient-days increased significantly for transitional age patients with cystic fibrosis, malignant neoplasms, and epilepsy, and for adults with cerebral palsy. Annual growth rates of charges increased for transitional and adult patients for all diagnoses except cystic fibrosis and sickle cell disease. CONCLUSIONS: The population of adults with diseases originating in childhood who are hospitalized at children's hospitals is increasing, with varying disease-specific changes over time. Our findings underscore the need for proactive identification of strategies to care for adult survivors of pediatric diseases.