Raised serum alkaline phosphatase (ALP) activity in rheumatoid arthritis (RA) has been reported, although its aetiology is not clear. In this paper we investigate whether synovial tissue is a possible source of raised ALP activity in RA. The activities and isozymes of ALP were determined in sera and synovial fluids from 22 RA and seven osteoarthritis (OA) patients. The expression of both protein and ALP mRNA in synovial tissue was investigated immunohistochemically and by reverse transcription (RT) PCR. ALP activity was higher in serum and synovial fluid from RA patients than in those from OA patients. In addition, the ratio of levels of bone-type ALP to those of liver-type ALP was significantly higher in synovial fluid than in serum from RA patients. Bone-type ALP was positive around the perivascular area and the subepithelial cells in the synovial tissue from RA patients. In contrast, the synovial tissue from OA patients exhibited no staining. The mRNA of bone-type ALP was detected in RA synoviocytes. In conclusion, ALP levels were elevated in both serum and synovial fluid from RA patients. Bone-type ALP derived from the synovial tissue may contribute to the raised activities of ALP in RA patients.