Aim of this retrospective study is to evaluate the effect of acute and late anterior cruciate ligament (ACL) reconstruction in patients with a combined injury of the ACL and the medial collateral ligament (MCL). All MCL injuries were treated non-operatively. In 27 patients (group I) we performed early ACL reconstruction (within the first 3 weeks after injury). The postoperative rehabilitation protocol included brace treatment for all patients over a period of 6 weeks. In 37 patients we performed late ACL reconstruction (after a minimum of 10 weeks). In this group initial non-operative MCL treatment (6 weeks brace treatment) was followed by a period of accelerated rehabilitation. Patients with late ACL reconstruction had a lower rate of loss of motion after finishing the postoperative rehabilitation programme and a lower rate of re-arthroscopies for a loss of extension (group I: 4 patients, group II: 1 patient). The difference in the mean quadriceps muscle strength (group I: 83.3%, group II: 86.3%) was not statistically significant. After a mean interval of 22 months, we saw no difference in the frequency of anterior or medial instabilities or in the loss of motion. The Lysholm score was significantly better in the group with late ACL reconstruction (group I: 85.3, group II: 89.9). The position on the Tegner activity scale decreased in both groups, to 5.5 in group I (preoperatively: 6.0) and to 5.6 in group II (preoperatively: 5.9). With regard to the lower rate of motion complications in the early postoperative period, the lower rate of re-arthroscopies, and the significantly better results in the Lysholm score, we prefer late ACL reconstruction in the treatment of combined injuries of the ACL and the MCL.