If posterolateral rotatory instability (PLRI) injury in patients with a torn anterior cruciate ligament (ACL) is not diagnosed and treated, ACL reconstruction can fail. We retrospectively evaluated the clinical outcome after reconstructions between 2002 and 2007 of both the ACL and the posterolateral corner (PLC) in 44 knees with combined ACL and PLC injuries. The median follow-up duration was 49 months (range, 24-68 months). ACL reconstruction employed autogenous hamstring grafts from the ipsilateral knee. For grade II PLRI, a posterolateral corner sling through the fibular head was placed obliquely from the anteroinferior aspect to the posterosuperior aspect using autogenous hamstring grafts of the contralateral knee. Clinical outcomes were evaluated using the OAK (Orthopadishe Arbeitsgruppe Knie) and IKDC (International Knee Documentation Committee) knee scoring systems. Anterior stability was measured on pull stress radiographs using a Telos stress device and the manual maximum displacement test using a KT-1000 arthrometer with the knee flexed 30 degrees. PLRI was classified according to varus and rotational instability preoperatively and at final follow-up. Median OAK scores improved from 71 points (range, 48-86) to 93 points (range, 75-100). Satisfactory IKDC results were achieved in 39 knees (89%). As for anterior stability, as measured by anterior stress radiography, mean side-to-side displacement difference dropped significantly from 6.9 +/- 1.9 preoperatively to 1.4 +/- 1.1 mm at final follow-up. Forty patients (91%) had the same or better rotational stability compared to the normal side. Varus stress radiographs showed mean side-to-side displacement differences dropped from 1.8 +/- 1.7 preoperatively to 0.4 +/- 0.8 mm at final follow-up. Thus, chronic ACL deficiency is often accompanied by grade II PLRI and can be treated successfully by arthroscopic ACL reconstruction paired with posterolateral reconstruction employing a single sling through the fibular tunnel and a hamstring tendon autograft.