Diagnostic criteria were established to improve the diagnostic accuracy of the meniscus lesion of the knee. Diagnostic criteria consist of combination of clinical scores and examination findings. Twelve findings probably occurring in meniscus lesion were selected and clinical scores were empirically assigned. Namely, they were such as the followings. 1. Locking phenomenon (30 points) 2. Snapping in movements (20 points) 3. Mass palpable on joint line in movement (20 points) 4. Locking-like phenomenon (15 points) 5. Giving-way (10 points) 6. Screw-home movement disturbance (10 points) 7. Quadriceps atrophy (10 points) 8. Clinical nomenclature test (10 points 9. Tenderness on joint line (5 points) 10. Full flexion disturbance (5 points) 11. Patella floating (3 points) 12. Pain in stepping upward and downward (1 point) Examination findings were classified into four groups according to their appearance as follows. 1. Typical...... Arthrogram shows abnormal line or pooling of contract medium and reveals the type of lesion in meniscus. Arthroscopy reveals lesion in meniscus directly. 2. Definitive...... Arthrogram shows abnormal line or pooling of contract medium, but does not reveal the type of lesion. Arthroscopy doesn't reveal lesion on meniscus directly but demonstrates abnormality occurring in meniscus lesion. 3. Suspective...... Arthrogram shows dullness of triangular corner or interruption of meniscus line, or deformity of meniscus image. Arthroscopy reveals discoloration or abnormal mobility of meniscus. 4. Nothing particular...... Arthrogram shows intact meniscus. Arthroscopy reveals intact meniscus. Diagnostic criteria, 1. When clinical score is more than 40 points, more than "suspective" results of clinical examination are required. 2. When clinical score is between 39 and 30 points, more than "definitive" results of clinical examination are required. 3. When clinical score is under 30 points, "typical" results of clinical examination are necessary. According to these criteria, 100 meniscectomies have been performed over 5 consecutive years. Since false positive results were obtained in none, the diagnostic value of the present criteria was 100%. This figure compares favorably with any values previously reported.