The incidence of an effusion in the knee in 155 consecutive elderly patients who had been operated on for a proximal femoral fracture was studied. The preoperative incidence had been 7.7 per cent in the ipsilateral knee and 1.3 per cent in the contralateral knee. Postoperatively, fifty patients (32.3 per cent) had an effusion on the ipsilateral side. In seven of them, the effusion had been present before the operation. All of the effusions subsided completely within three weeks after the operation. Results of the laboratory analysis of a specimen of the effused material from eight patients who were chosen at random showed non-inflammatory fluid. Probably the effusions were traumatic in origin, and it is likely that they were a response to stresses that had been incurred during the operation or at the time of fracture.