I analyzed insurance claims for injuries sustained in high-school football over a three-year period in six western states. Nineteen and one-half per cent of the players sustained at least one injury per season, of which 12.7 per cent were knee injuries. Follow-up questionnaires completed by 529 (81 per cent) of the injured players showed that 394 had been diagnosed as having ligament injuries; eighty-eight, internal derangement of the knee; and forty-seven, patellar injuries. Twenty-six per cent of the patients with ligament injuries had further injury to the involved knee. Fifty-nine per cent of the patients with internal derangement of the knee had subsequent injury to the same knee. Eighty-seven per cent of the patients with patellar injuries had a subsequent injury to the involved knee and an increased incidence (34 per cent) of injury to the contralateral knee.