We conducted a study to analyze nationally representative data on patient and health care system characteristics and in-hospital outcomes associated with primary and revision total hip arthroplasties in the United States. Between 1990 and 2004, there were an estimated 2,748,187 hospital discharges after total hip arthroplasty. The risk factors we identified for procedure-related complications and in-hospital mortality included revision procedures, increased age, and male sex. Compared with smaller hospital capacity (number of beds), large hospital capacity was associated with a decreased odds ratio for complications but an increased risk for in-hospital mortality. Additional studies are warranted to determine causal relationships.