Several authors have identified on admission haemoglobin level as the most useful predictor of transfusion risk in patients with a hip fracture. A low postoperative haemoglobin unexplained by perioperative blood loss is not uncommon in these patients. The drop in haemoglobin after re-hydration prior to surgery should depend not only on the degree of dehydration but also on the amount of blood lost in the fracture. We could find no study in the English literature estimating the magnitude of this fall in haemoglobin after re-hydration prior to surgery. We conducted a prospective study to estimate the magnitude of fall in haemoglobin after re-hydration prior to surgery by repeating the full blood count after at least 12h of preoperative fluid resuscitation in 127 patients with hip fracture (75 consecutive at one centre and 52 consecutive at another). The average preoperative drop in haemoglobin was 2.23gram/decilitre (g/dL) (p-value=0.00) in subtrochanteric fractures, 1.1g/dL (p-value=0.001) in intertrochanteric fractures and 0.7g/dL (p-value=0.02) in intracapsular fractures. Fifteen patients with a haemoglobin level >9g/dL on admission were found to have a haemoglobin level <9g/dL on repeat test and were prevented from going to theatre without arrangements for perioperative transfusion during this study. Their predicted average postoperative haemoglobin without perioperative blood transfusion was calculated to be 6.5g/dL. The on admission haemoglobin level was found to be falsely reassuring and could lead to a very low postoperative haemoglobin level. This could prove to be dangerous for many patients especially if remains undetected for several hours. We recommend that all patients with subtrochanteric fractures, and all patients with intertrochanteric or intracapsular fractures with a haemoglobin of less than 12g/dL on admission have a repeat haemoglobin level performed prior to their surgery.