A review of reports on nonoperative treatment of acute, isolated anterior cruciate ligament (ACL) tears shows a large variability in results. This may be caused by variations in study designs. To our knowledge, no prospective study on isolated ACL tears diagnosed through arthroscopy has been published. We are reporting the short-term results of a prospective study of 29 nonoperatively treated acute ACL injuries. All patients had a history of trauma, with contact sports resulting in 20 of 29 injuries. The mean age was 25 years. The mean follow-up was 33 months. Of the 29 patients, 11 required ligament reconstruction in the follow-up period because of considerable instabilities. Only two patients kept their activity level. All but two patients had a positive pivot shift. The reconstructed group was younger than the nonoperated group, had a higher preinjury level of activity both in sports and daily life, and in general had a desire to stay at the preinjury level. The nonreconstructed group had a lower preinjury activity level and was willing to reduce their postinjury level. When suggesting nonoperative treatment to a young patient with an isolated ACL tear, the surgeon should emphasize the importance of abandoning contact sports and obtaining good muscle strength. A young patient in a knee-demanding profession or insisting on continuing knee-demanding sports should have an early reconstruction carried out.