A prospective study of the serum levels of unfractionated creatine phosphokinase (CPK) in 69 consecutive elderly patients undergoing surgery for hip fracture is reported. Serum unfractionated CPK levels were measured on admission, on the evening following surgery and daily for the first five days post-operatively. All of the CPK levels measured on admission were within the normal range for this laboratory. A gradual rise in CPK levels followed surgery. On the evening following surgery 75% of values were greater than the upper limit of the normal reference range. The peak values were seen on day 1 post-operatively but 25% of values were still less than the reference maximum. A gradual decline was seen after this and at day 5 post operatively 50% of values were within the normal reference range. A single unfractionated CPK determination is not of diagnostic benefit in the post-operative period in elderly patients with hip fracture. However, patients with intracapsular and intertrochanteric fractures do not show elevation of unfractionated CPK levels prior to surgery.