OBJECTIVE: To compare health care use and costs for youth with chronic health conditions before and after transfer from pediatric to adult health care services. METHODS: Youth born in Ontario, Canada, between April 1, 1989, and April 1, 1993, were assigned to 11 mutually exclusive, hierarchically arranged clinical groupings, including “complex” chronic conditions (CCCs), non-complex chronic conditions (N-CCCs), and chronic mental health conditions (CMHCs). Outcomes were compared between 2-year periods before and after transfer of pediatric services, the subjects’ 18th birthday. RESULTS: Among 104 497 youth, mortality was highest in those with CCCs, but did not increase after transfer (1.3% vs 1.5%, P = .55). Costs were highest among youth with CCCs and decreased after transfer (before and after median [interquartile range]: $4626 [1253–21 435] vs $3733 [950–16 841], P < .001);Costs increased slightly for N-CCCs ($569 [263–1246] vs $589 [262–1333], P < .001), and decreased for CMHCs ($1774 [659–5977] vs $1545 [529–5128], P < .001). Emergency department visits increased only among youth with N-CCCs (P < .001). High-acuity emergency department visits increased CCCs (P = .04) and N-CCCs (P < .001), but not for CMHC (P = .59), who had the highest visit rate. Among the 11 individual conditions, costs only increased in youth with asthma (P < .001), and decreased (P < .05) in those with neurologic impairment, lupus, inflammatory bowel disease, and mood/affective disorders. CONCLUSIONS: Pediatric transfer to adult care is characterized by relatively stable short-term patterns of health service use and costs among youth with chronic conditions.