An anterior cruciate ligament tear before or early in an athlete's season presents a treatment dilemma: surgically repair the ligament and end the season, or use rehabilitative exercises and bracing to quickly return the athlete to play. We conducted a prospective study of 43 athletes (44 acute tears) over 44 months to determine criteria for early return to play and if an early return is safe. All patients had an acute injury in a previously normal knee, a positive Lachman test, and KT-1000 arthrometry indicating ligament abnormalities. Magnetic resonance imaging of each injured knee showed an interior cruciate ligament tear but no meniscal tear. Thirty patients (31 tears) returned to play with rehabilitation and a brace at an average of 5.7 weeks after injury: Only 12 patients returned to their sports without recurrent buckling of their injured knees; 18 patients (19 knees) had recurrent buckling during play. Thirteen patients could not return to play. Patients were observed until they 1) had ligament reconstruction (29 patients, 29 tears), 2) gave up their sports because of instability but did not elect surgery (3 patients), or 3) returned to play in a brace and declined surgery (11 patients, 12 tears). All who elected reconstruction experienced recurrent knee buckling. We found 23 meniscal tears (17 knees) in the 29 patients undergoing reconstruction.