We report the histopathologic features of the knee bone and synovium and the cytologic features of the synovial fluid from a patient with alpha-mannosidosis. The synovium showed marked papillary hyperplasia with infiltration of foamy histiocytes containing periodic acid-Schiff-positive, diastase-resistant material. Severe degenerative changes were seen in the knee bone. The synovial fluid showed increased numbers of macrophages containing periodic acid-Schiff-positive, diastase-resistant material. The differential diagnostic considerations in the synovial fluid are also discussed.