This study compares blood loss at total hip replacement in obese and non-obese patients. We made a prospective study of intra-operative and postoperative blood loss in 80 consecutive primary cemented hip replacements. Patients' obesity was classified according to body mass index (BMI). Overall mean total blood loss was 1050 ml. Obese patients (BMI > 30) bled significantly more (P < 0.0001) than those of optimal weight (BMI < 26), whereas those overweight (BMI 26-30) did not. The mean excess blood loss in obese patients was 380 ml (95% confidence interval, 200-560 ml). At a time when the prevalence of obesity is increasing, this study quantifies the risks of greater blood loss with respect to obesity and aids informed consent.