The purpose of this study was to prospectively evaluate the prevalence of abnormal magnetic resonance imaging scans of the knees of asymptomatic subjects. A prospective analysis of magnetic resonance imaging to arthroscopic findings in symptomatic knees was also performed. The prevalence of meniscal tears found in asymptomatic knees was 5.6% (medial meniscus, 1.9%; lateral meniscus, 3.7%). Other abnormal findings included a prevalence of 1.9% for degenerative changes of the medial femoral condyle and 3.7% both for ganglion cysts and patellofemoral joint articular cartilage degenerative changes. There was also a prevalence of 24.1% of Grade II signal changes of the posterior horn of the medial meniscus. Statistical comparison of our results to previous studies revealed that the magnetic resonance imaging scan readings on the asymptomatic knees in this study were accurate and lesions were correctly identified. We recommend that clinicians match clinical signs and symptoms with magnetic resonance imaging findings before instituting surgical treatment because of a 5.6% prevalence of meniscal tears in the asymptomatic population. The significance of the high percentage of posterior horn medial meniscal Grade II signal changes is unknown.