No representative data exist on the risks of adverse outcomes of total hip arthroplasty (THA) in the United Kingdom. A prospective observational study of unselected THA operations was carried out in 5 U.K. regions. Adverse outcomes were assessed from the hospital case notes and general practitioners of 1,100 randomly selected patients and from 7,151 patient-completed questionnaires 3 and 12 months after THA. Three-month mortality was 0.4% to 0.7%. Dislocation and thromboembolic complications were about 3% and 4%. Perioperative fracture, sciatic nerve palsy, aseptic loosening, and revision each had a risk of < or = 1%. At 1 year, 2.6% of patients had undergone another operation on the same hip, 11% reported moderate or severe pain in the operated hip, 23% had severe walking restriction, and 11% were dissatisfied with the operation. Patients and surgeons in the United Kingdom should have access to this information when making a decision about THA.