PURPOSETo evaluate the incidence of meniscal tears in patients with chronic anterior cruciate ligament (ACL)-deficient knees, and to determine the influence of posterior tibial slope (PTS) on medial meniscal tears in ACL-deficient knees.MATERIALS AND METHODSWe reviewed 174 patients (174 knees) with a mean age of 30.7 years who underwent ACL reconstruction for chronic ACL tears. We divided the patients into two groups: low group (135 knees with a PTS<13°) and high group (39 knees with a PTS≥13°).RESULTSThe incidence of medial meniscus tears was 44% (77/174), and that of lateral meniscus tears was 35% (61/174). The mean PTS in patients with medial meniscal tears was 11.4°±3.0°, whereas that in patients without medial meniscal tears was 9.8°±2.4°. The incidence of meniscal tears was 57.8% (78/135) in the low group and 89.7% (35/39) in the high group (p<0.001).CONCLUSIONSOur data demonstrate that PTS≥13° is a risk factor for secondary medial meniscal tears in ACL-deficient knees. So, we suggest that PTS is one of the considerations for determining early ACL reconstruction to prevent secondary meniscal tears.