PURPOSE: Anterior cruciate ligament (ACL) injuries often lead to associated injuries of the posterior horn of the lateral meniscus (PHLM). Arthroscopic, assessment of PHLM instability may be difficult in the absence of a visible meniscus damage. The main objective of this prospective multi-center study was to compare the ability of the probing and aspiration tests to identify PHLM instability in a population of patients undergoing ACL reconstruction (ACLR) and a control group of patients with an intact ACL undergoing knee arthroscopy. METHODS: A prospective case-control analysis was performed in three sports medicine centers. One-hundred and three consecutive patients operated for a primary isolated ACLR without structural lateral meniscus damage other than a root tear were included. They were compared to a control group of 29 consecutive patients who had a knee arthroscopy with an intact ACL and no structural lateral meniscus lesion. The probing and aspiration tests were consecutively executed according to previously published methods. RESULTS: In the control group, no lateral meniscus lesions were visualized during arthroscopy, and both probing and aspiration tests were negative in all patients. In the group of ACL-injured patients, a Forkel type I-III posterolateral meniscus root tear (PLMRT) was found in 12 patients (12%). In this subgroup, the probing test was positive in 4/12 patients (33%) and the aspiration test in 5 additional patients (75%). In 15 patients (15%), an elongation of the posterior root of the lateral meniscus (defined as type IV PLMRT as an addendum to the Forkel classification) could be observed during arthroscopy. In this subgroup, only 1 patient displayed a PHLM instability with the probing test (7%), whereas the aspiration test was positive in 13/15 patients (87%). In the remaining 76 patients (74%), no structural lesion of the PHLM could be identified. Nevertheless, an instability of the PHLM could be identified in 8 of them (11%) with the probing test, and the aspiration test was positive in 2 additional knees (13%) of this apparently normal subgroup. Altogether, in the entire ACL injury cohort, a positive probing test was observed in 13/103 patients (13%) and a positive aspiration test in 32/103 knees (31%) (p \textless 0.01). CONCLUSION: Careful observation and examination of the PHLM with the aspiration test revealed a substantial amount of previously undiagnosed lateral meniscus instabilities in ACL-injured knees. The prevalence of PHLM instability as evaluated by the aspiration test was high (31%). The aspiration test was superior to the probing test in detecting an instability of the PHLM in a population of ACL-injured patients. LEVEL OF EVIDENCE: II.