OBJECTIVES: This study evaluated the correlation between patient comorbidity status and total hospitalization length of stay (LOS) and cost for total knee arthroplasty (TKA), total hip arthroplasty (THA), and one to three level lumbar spinal fusion procedures. METHODS: Using the Premier Database from 1/1/2008 – 6/30/2014, we identified adults 18+ years old with a primary procedure of TKA, THA, or spinal fusion.  Patients with multiple orthopedic procedures during the hospitalization, fracture diagnoses, and revision procedures were excluded.  Mean hospital length of stay (LOS) was compared by Charlson Score with ANOVA.   Generalized linear models (GLM) – controlling for age, sex, region, hospital size, academic status, payor, and procedure year – predicted the incremental total hospitalization cost among the sickest patients (Charlson ≥ 3) vs healthy controls (Charlson zero). RESULTS: