In a prospective study, 90 consecutive patients with total midstructural tears of the anterior cruciate ligament (ACL) were assigned at random to surgical (Group I) or conservative (Group II) treatment. Within 18.2 months of operation, 95% of the patients in Group I and 11% of those in Group II had a stable knee. The mean knee function score in Group I was 89 points, and 75% achieved more than 84 points (good or excellent). In Group II the mean score was 85; only 53% achieved more than 84 points (p less than .05). Group II patients showed greater mean quadriceps strength than those in Group I. The ability to perform a one-leg jump and to run a figure eight was similar in both groups. Early primary suture of the acutely torn ACL usually resulted in a stable knee, whereas conservatively treated patients showed knee instability. At the 18-month interval, however, the patients' functional performance seemed to be comparable in the two groups.