Hematoma following primary total hip arthroplasty (THA) can require a return to the operating room. The purpose of this study was to uncover risk factors for hematoma and how it affects the outcome of THA. This case–control study identified 38 patients requiring reoperation due to hematoma following THA between 2000 and 2007. The 38 patients were matched with 117 patients without hematoma. The mean follow-up was 4.1 years (range, 2.1–9.6). Multivariate regression showed that blood loss, administration of fresh frozen plasma and Vitamin K, perioperative anticoagulation and hormonal therapy were independent predictors for hematoma formation. Chronic anticoagulation and autologous blood transfusion were independent risk factors for mortality. Hematoma itself was found to be an independent risk factor for adverse outcomes, increasing morbidity and mortality, despite adequate treatment.