The incidence and prognostic significance of fractures of acrylic cement related to the stem of a femoral head prosthesis in total hip replacement are examined. These fractures are demonsfrated when the cement has been rendered radio-opaque by the addition of barium sulphate. One and a half per cent of the radiographs of 6,649 patients showed these fractures, which were sometimes associated with subsidence of the prosthesis. Fracture of the cement was usually evident at the six-month post-operative review, if it occurred at all. This radiological complication was devoid of symptoms in the majority of cases and tended to occur in patients with excellent functional recovery. In a minority of patients pain in the thigh during the first six months seemed likely to be explained by this fracture. Slight subsidence of the prosthesis in the cement bed appeared to result in a new and final position of stability. The prognosis was very good; only when separation of the fracture exceeded about 4 millimetres was the prognosis doubtful, in which case a chronic deep infection might be suspected. Possible mechanical and structural causes of fracture of the cement are discussed.