The morphological distribution of hyaluronan in normal and diseased synovium has been determined using a probe derived from the hyaluronan binding region of cartilage proteoglycan core protein. Normal synovium showed hyaluronan surrounding the lining layer cells with little in deeper layers. Rheumatoid synovium showed intense staining for hyaluronan throughout the tissue, notably associated with blood vessels and areas of dense cellular infiltration. Osteoarthritic tissues varied according to the degree of infiltration present, with inflamed specimens closely resembling rheumatoid tissue. The distribution of hyaluronan in diseased synovium suggests a role in aspects of the inflammatory process such as angiogenesis and cell traffic.