Three hundred one unconstrained meniscal arthroplasties were observed for as long as nine years, during which time 25 (8.3%) failed. Risk factors were sought by comparing the distributions of several preoperative variables in the group that failed with the group that was successful. Age, weight, the magnitude or direction of preoperative deformity, and the presence of postoperative malalignment were all without effect on the outcome of the operations. Knees with rheumatoid arthritis had a 95% survival rate at six years. Knees with osteoarthrosis had an equivalent survival rate of 83%. Knees in which the anterior cruciate ligament (ACL) was normal had a survival rate of 95% at six years; those in which the ligament was damaged or absent had an equivalent survival rate of 81%. Successful reconstruction of a knee with an unconstrained meniscal implant requires the presence and the preservation of an intact ACL.