Histopathological studies have suggested that spontaneous degeneration of knee menisci predisposes to symptomatic tear. We used magnetic resonance imaging (MRI) to study noninvasively both 20 patients who have had chronic symptoms over 1 year with documented unilateral meniscal tears in one knee and 15 normal controls. A scoring system for MRI signal changes was developed, and differences between the two groups were tested for significance by unpaired t-test. The menisci were evaluated on Tl weighted sagittal image and grading of meniscal signals were from Grade 0 to Grade III. 1. Grade 0 is no signals within homogenous black menisci. 2. Grade I is globular or irregular and does not extend to the articular surface. 3. Grade II is linear and does not extend to the articular surface. 4. Grade III extends to the articular surface and represents a meniscal tear. The mean scores of the lateral meniscus of the same knee with medial meniscal tear are significantly higher than both those of lateral meniscus in normal controls (p=0.0361) and the mean scores of the medial meniscus of the same knee with lateral meniscal tears are significantly higher than those of medial meniscus in normal controls (p=0.0423). These results support the hypothesis that chronic unilateral meniscal tear influences on degeneration of asymptomatic meniscus in the same knee.