We retrospectively reviewed the records of fifty-two patients who had had a rupture of the anterior cruciate ligament between the ages of forty and sixty years, to determine the results of aggressive non-operative treatment. We were able to locate and re-examine thirty of these patients (mean age, forty-six years) after a mean duration of follow-up of seven years (range, five to thirteen years), and to assess the clinical, radiographic, and functional results. The mean score, according to the scale of Lysholm and Gillquist, was 82 points; eight of the eleven patients who had combined ligamentous injuries had a score of less than 84 points (symptoms with daily activities). Thirteen substantial reinjuries had occurred in eleven patients (37 per cent) during the follow-up period. Twenty-nine patients (97 per cent) had a grade-2 or 3 Lachman test, and a positive pivot-shift test was elicited in twenty-five patients (83 per cent). Plain radiographs revealed minimum or no changes in twenty-six patients (87 per cent). Magnetic resonance imaging in nine patients revealed scarring of the remnant of the anterior cruciate ligament to the posterior cruciate ligament in six. The mean difference in anterior-posterior laxity between the injured knee and the normal, contralateral knee, as measured with the KT-1000 arthrometer, was five millimeters at twenty pounds (eighty-nine newtons). Twenty-five (83 per cent) of these thirty middle-aged patients, who had had guided rehabilitation and had modified activity, had a satisfactory outcome without an operation. However, a few patients, who had combined instabilities and who wished to resume competitive sports activity that required pivoting, were dissatisfied. Such patients may need operative reconstruction to achieve their goals.