The majority of previous studies on partial ruptures of the anterior cruciate ligament (ACL) include a relatively large proportion of knees with associated intra-articular injury or collateral ligament tear that contributes to an increase in the symptoms of instability and further deterioration of knee function. In the present study only patients with isolated, partial ruptures of the ACL were evaluated. Fifty-six patients with one injured knee were examined after a median of 5.3 (range 2.0-12.7) years using the IKDC evaluation form, Lysholm knee function score and Tegner activity score. Of the 56 knees, 6 underwent autologous reconstruction due to early progression to complete rupture. Of 34 knees evaluated for laxity, 25 had a negative Lachman test and 7 a positive (+) Lachman. In 2 knees a Lachman ++ result and a positive pivot shift were found. With instrumented laxity testing 24 knees had 2 mm or less difference in laxity compared with the contralateral uninjured knee. The largest side-to-side difference in knee laxity was 4.5 mm. Lysholm score was median 86 (range 52-100) points, and 62% had good or excellent knee function. A significant decline in activity was seen. Only 10 patients (30%) resumed their preinjury activities. We find that the majority of patients with an isolated, partial rupture of the ACL have an acceptable knee function and a stable knee after a median 5 years follow-up. There is, however, a marked reduction in activity.