Wear of articular cartilage usually occurs at flexion zones (30°–60°) of the femorotibial articulation, which is demonstrable on the 45°posteroanterior (PA) weight-bearing view. The authors measured and evaluated the cartilage space on the non weight-bearing AP view, AP extension weight-bearing view and 45°PA flexion weight-bearing view, comparing operative findings in cases of 18 osteoarthritis, 7 rheumatoid arthritis and 2 spontaneous osteonecrosis of the lateral femoral condyle, who underwent total or hemiarthro plasty (1 case) of the knee. The results were as follows. l. Operative findings of 11 out of 18 osteoarthritis (61.1%) showed cartilage defect of middle 3/4–4/5 of the medial tibial condyle (sparing finger tip sized anterior & posterior edge) and corresponding femoral condyle. The cartilage space on the AP non weight-bearing, AP extension weight-bearing and 45°PA flexion weight-bearing view were normal to 20% decrease (4.1–5.0mm remained), 60–80% decrease (1.1–2.0mm remained) and 80–100% decrease (0–1.0mm remained), respectively, showing one grade increased cartilage space narrowing on the 45°PA flexion weight-bearing view comparing to AP extension weight-bearing view. 2. Uniform cartilage defect of medial compartment was found in 7(38.9%) osteoarthritic knees. The cartilage space on the non weight-bearing AP view were normal. Both AP & PA weightbearing views showed 60–80% decrease (1.1–2.0mm remained) in 3 knees & 80–100% decrease (0–1.0 mm remained) in 4 knees without any difference between the two views. 3. Uniform cartilage space narrowing of both medal & lateral compartment was noted in 7 rheumatoid knees. The cartilage space on the AP non weight-bearing view were 20–40% decrease (3.1–4.0mm remained) in all knees. The AP extension & 45°PA flexion view showed 40–60% decrease (2.1–3.0mm remained) in 2 knees, 60–80% (1.1–2.0mm remained) in 5 knees respectively without any difference between two groups. 4. The anterior horn of menisci was always intact. The presence of menisci didn't affect the radiological cartilage space. 5. The 45°PA flexion weight-bearing view functioned as a tunnel view showing bone defect and less than 20% decrease of cartilage space in cases of the spontaneous osteonecrosis of the lateral femoral condyle. From our results it was concluded that the radiologic cartilage space could be more accurately demonstrated with 45°PA flexion weight-bearing view in addition to the conventional AP extension weight-bearing view.