OBJECTIVES: To assess length of stay (LOS) and costs attributable to ADHD among adolescents hospitalized with a primary diagnosis of ODD.  METHODS: Patients 12-17 years old with a primary diagnosis of ODD (ICD-9-CM code 313.81) were selected from the 2000 to 2006 Healthcare Cost and Utilization Project Nationwide Inpatient Sample (HCUP-NIS). Patients with a diagnosis of ADHD (ICD-9-CM codes 314.00 and 314.01) comprised the study cohort and patients without an ADHD diagnosis comprised the control cohort. Study measures included demographics, hospital characteristics, admission source, discharge disposition, LOS, and costs. Generalized linear models accounting for the HCUP-NIS survey design were undertaken to adjust LOS and cost estimates. RESULTS: A total of 7,404 and 18,039 patients met the inclusion criteria for the study and control cohorts, respectively. Patients in the study cohort were 6.8 months younger than patients in the control cohort (13.8 versus 14.4 years). A higher percentage of patients in the study cohort were male (71.3% versus 45.2%) or had Medicaid (57.1% versus 48.6%) compared to the control cohort. In both cohorts, the ER was the most common admission source, approximately 90% of patients had their discharge disposition recorded as routine, and most patients were treated in urban, teaching, or large bedsize hospitals. The study cohort had longer LOS and higher costs versus the control cohort (mean [SE] 9.48 [0.89] days and $8241 [$1356] versus 7.90 [0.59] days and $6466 [$709]). Regression analyses found the study cohort had significantly longer LOS and higher costs versus the control cohort (by 2.5 days and $1338). CONCLUSIONS: Patients hospitalized with a primary diagnosis of ODD and a secondary diagnosis of ADHD had significantly longer LOS and higher costs compared to patients with ODD but without ADHD. Clinicians and health care decision-makers should be aware of the impact ADHD has on inpatient stays among patients with ODD.