Fibrous tissue which regenerated on the acetabular arthroplasty surface was obtained from a 52-year-old woman who underwent total hip replacement after cup arthroplasty. The histological features of this newly formed fibrous tissue and expression of keratan sulfate, which is a characteristic matrix component of articular cartilage, were studied. Microscopic observation revealed that the arthroplasty surface consisted mainly of fibrous tissue which did not show metachromasia with toluidine blue staining, but there were many nodular structures communicating with the bone marrow. Immunostaining for keratan sulfate revealed clear positive staining around the cells of the nodular structures communicating with the bone marrow, while only weakly positive staining was observed in the superficial layer of the loose fibrous tissue. The present study revealed marked formation of articular cartilaginous tissue in areas having good communication with the bone marrow, which indicates that maintenance of this communication may be necessary to improve the outcome of cup arthroplasty.