Between 1975 and 1983, fifty-seven patients were treated at Sunnybrook and Harborview Medical Centers with immediate internal plate fixation of an open diaphyseal fracture of the forearm. Fifty patients were available for follow-up, which ranged from one to nine years and averaged three years. The injuries were classified on the basis of the extent of soft-tissue injury as defined by Gustilo and Anderson, and consisted of twenty Type-I injuries, nineteen Type-II injuries, and eleven Type-III injuries. The complications included deep infection in two patients and non-union in six. The functional results were excellent or good in 85 per cent of the series. This study demonstrates that immediate stable plate fixation is a beneficial method of treatment of open fractures of the forearm. The results are related to the severity of the initial soft-tissue injury and the surgical technique. Autogenous cancellous bone-grafting at the time of closure of the wound in comminuted fractures in which interfragmental compression cannot be obtained is recommended.