We sought to determine prospectively the natural history of acute, isolated, nonoperatively treated posterior cruciate ligament injuries in athletically active patients. The study population consisted of 133 patients (average age, 25.2 years at time of injury). All patients completed a subjective questionnaire each year for an average of 5.4 years (range, 2.3 to 11.4). Sixty-eight of the 133 patients returned to the clinic for long-term follow-up evaluation. Objectively, physical examination revealed no change in laxity from initial injury to follow-up. No correlation was found between radiographic joint space narrowing and grade of laxity. The mean modified Noyes knee score was 84.2 points, the mean Lysholm score was 83.4, and the mean Tegner activity score was 5.7. Patients with greater laxity did not have worse subjective scores. No correlation was found between subjective knee scores and time from injury. Regardless of the amount of laxity, half of the patients returned to the same sport at the same or higher level, one-third returned to the same sport at a lower level, and one-sixth did not return to the same sport. Results of this study suggest that athletically active patients with acute isolated posterior cruciate ligament tears treated nonoperatively achieved a level of objective and subjective knee function that was independent of the grade of laxity.