Optimized resource allocation, reimbursement negotiations, and provider comparisons hinge on an understanding of the drivers of healthcare costs. Indices of comorbid illness may be useful for stratifying patients based on cost. Total hospital cost was analyzed for 1 surgeon's hip arthroplasty patients (June 1998-March 2001). Three scales of health status were selected as independent predictors. One thousand ninety-two hip arthroplasty inpatient stays were evaluated. The median total hospital cost was 14,011 dollars. An increasing burden of comorbid illness as measured by the All Patient Refined Diagnosis Related Group Severity of Illness scale and the modified Charlson Comorbidity Index was significantly associated with increasing hospital cost. Comorbid illness is associated with cost; scales may be used to stratify patients based on risk of high cost care.