Sixty-two patients with post-traumatic radiologically visualized effusion in the elbow joint, apparently without bony damage, were randomized to either 1 week of immobilization in a plaster or immediately instructed in active exercises without any immobilization. They were reexamined by an orthopaedic surgeon weekly until recovery. New radiographs were taken after 1 week. Reevaluation of the radiographs by a radiologist revealed seven 'false-positive' effusions, i.e. neither effusion nor fracture, and 21 missed fractures. All but five missed fractures, continued in the study. Thirty patients started immediate active exercises instructed by the surgeon and 27 were immobilized. The 'active exercise' group had a significantly shorter recovery time (one week vs two weeks, P < 0.05). The presence of missed fractures did not influence the result and all patients recovered fully. Also four of the excluded patients with missed fractures recovered fully. One patient with missed fracture dropped out from follow-up. We recommend that an apparently isolated post-traumatic effusion in the elbow joint is treated with immediate active exercises followed by a clinical reexamination after one week supplemented with new radiographs if there is unsatisfactory clinical progress.