We examined the natural history of arthroscopic medial meniscectomy in knees with an isolated meniscal injury by reviewing 317 of 894 cases following medial meniscectomy. At the time of the initial surgery none of the knees had been operated on, and there was no evidence of ligament injury. The patients were reviewed clinically and radiologically after a mean of 11.5 years (range 10-15). The knee was considered "normal" or "nearly normal" by 91% of patients. In 218 patients the contralateral knee was asymptomatic without history of operation or significant injury and could be used as control for comparison. Radiology showed 22.4% greater excess prevalence of joint space narrowing in the operated compared to the control knee. The factors predisposing to a poor radiological result were age above 35 years, the presence of medial compartment cartilage degeneration at the time of the first arthroscopy, resection of the posterior one-third of the meniscus, and meniscal rim resection. Preoperative participation in sport was a predictor of a better outcome.