The knees of 72 patients with unilateral anterior-cruciate- ligament (ACL) injury were analyzed before ACL reconstruction as well as by follow-up arthroscopy on the day of staple removal. At ACL reconstruction 31 lateral menisci and 40 medial menisci were found to be normal. 28 lateral menisci and 24 medial menisci were treated surgically, while 13 lateral menisci and 8 medial menisci with small or incomplete meniscal tearing were not treated. At follow-up arthroscopy there were 3 new cases of lateral meniscal tearing and 3 new cases of medial meniscal tearing in the groups diagnosed as normal prior to surgery. Two of the 13 cases with small or incomplete lateral meniscal tearing required resection, 8 healed and the other 3 demonstrated no progressive change. Four of the 8 cases with small or incomplete medial meniscal tears healed, 3 exhibited no progressive change and one required surgical treatment. There was no correlation between meniscal tearing and knee instability as indicated by a positive Lachman test or a positive pivot shift sign. The results of the present study indicate that ACL reconstruction prevents progressive changes in meniscal tears and will prevent secondary osteoarthritis, and that some small tears of the lateral meniscus require no surgical treatment.