Background: One of the causes of failure after unicompartmental knee arthroplasty (UKA) for varus osteoarthritis is the progression of osteoarthritis in the lateral compartment. Bone formation within the articular cartilage has been reported as a possible cause of the deterioration of the osteoarthritis. However, its prevalence and histology are still uncertain. The purpose of the current study is to investigate bone pieces obtained from the lateral compartment of the knee during total knee arthroplasty (TKA) radiographically and histopathologically, and to examine the bone formation within the articular cartilage in detail. Methods: Thirty-four consecutive patients, who underwent TKA for varus osteoarthritis of the knee, were included in this study. There were 6 males and 28 females and the mean age at the surgery was 74 years (range 63-85 years). Fifty-seven and 99 sagittal slabs were acquired from the resected bone pieces of the distal lateral femoral condyle and lateral tibial plateau, respectively. These slabs were investigated by soft X-ray radiographs and histopathological analyses stained with hematoxylin-eosin and elastica-Masson. Results: On soft X-ray radiographs, bone formation (in which bone trabeculae were clearly detected) within the articular cartilage was observed in 8 (14.0 %) of 57 femoral slabs and 7 (7.1 %) of 99 tibial slabs. Histopathological analyses revealed that the bone which had formed comprised normal trabeculae and fatty marrow. There was rich vascularity in the cartilage around the formed bone. Conclusions: The current study is the first to describe the prevalence, location, and histopathological findings of bone formation within the articular cartilage of the lateral compartment of varus osteoarthritic knees. Since the thickness of the cartilage covering the bone formation is less than that without it, it may result in the deterioration of the lateral compartment after UKA.