We evaluated the results of anterior cruciate ligament (ACL) reconstruction using an Achilles tendon allograft in revisions and in patients older than 30. Results from 23 consecutive patients (mean age, 43 years) who underwent ACL reconstruction with fresh-frozen, irradiated (22/23) Achilles allografts were retrospectively reviewed. Seven cases were revisions. Patients were evaluated with physical examination, questionnaires, and x-rays. Twenty of the 23 patients were evaluated a mean of 28 months after surgery. There were 5 failures (21%); 3 acute failures were not evaluated at follow-up. One patient had an infection that required graft removal, 2 patients had mechanical failure of the grafts, and 2 had displacements of more than 5.5 mm as measured with a KT-1000 arthrometer. The 18 clinically successful cases had full motion, no thigh atrophy, and no effusion. Pivot shift scores were 55% A and 45% B on the International Knee Documentation Committee (IKDC) scale. Lachman scores were 40% A, 55% B, and 5% C on the IKDC scale. The KT-1000 difference was a mean of 2.9 mm at final follow-up. However, knees loosened a mean of 4.5 mm from the immediate postoperative measurements (P<.0001). Mean Lysholm and Tegner scores were 86.8 and 5.2, respectively. Tibial tunnel diameter increased by 3.1 mm on anteroposterior x-rays and 3.0 mm on lateral x-rays. Five patients developed mild medial compartment arthritis. Four of the 5 grafts with failures were from donors older than 40. Postoperative complications included deep vein thrombosis and inflammatory effusion (white blood cell count, 15,000). Twenty-one percent of ACL reconstructions with Achilles tendon allografts failed. Grafts deemed successful still had significant loosening at final follow-up. Allografts from donors older than 40 may have played a role in these failures. From the data in this study, it appears that surgeons should scrutinize the source of the allograft tissue and the age of the donor.