PURPOSE: To describe the most prevalent and costly inpatient hospitalizations in a national cohort of privately insured young adults since the Affordable Care Act. METHODS: Cross-sectional study of a national administrative data set of privately insured young adult (18-30 years) beneficiaries hospitalized from January 2012 to June 2013. The most prevalent diagnosis categories for young adult hospitalizations are presented as percentages of all young adult hospitalizations by gender and age group (18-21, 22-25, and 26-30 years). Mean and median out-of-pocket costs by diagnosis category and gender are calculated based on deductible, copay and coinsurance payments. RESULTS: We analyzed 158,777 hospitalizations among 4.7 million young adult beneficiaries; young adults accounted for 18.3% of privately insured hospitalizations across all ages. Top diagnoses for young adult female hospitalizations were pregnancy related (71.9%) and mental illness (8.9%). Top diagnoses for young adult male hospitalizations were mental illness (39.3%) and injuries and poisoning (14.0%). Mean and median total out-of-pocket costs for any young adult hospitalization were $1,034 and $700, respectively (mean deductible payment = $411). The most expensive out-of-pocket hospitalizations were for dermatologic diseases (e.g., skin infections) with means of $1,306 for females and $1,287 for males. CONCLUSIONS: This study establishes a baseline for the ongoing assessment of the most common and costly hospitalizations among privately insured young adults in the United States under the Affordable Care Act. The substantial burden of potentially avoidable hospitalizations (e.g., mental health, injury, and poisonings) supports resource allocation to improve outpatient services, mental health access, and public health prevention strategies for young adults.