Increased concentration of serum alkaline phosphatase (ALP) is a common feature in rheumatoid arthritis (RA), although its origin remains unclear. The aim of this study is to analyze the origin and clinical significance of the elevated ALP value in RA. In 123 RA and 63 age- and sex-matched OA (osteoarthropathy) patients, concentrations of total ALP and its isozymes in serum and synovial fluid were studied. Serum CRP, Fe, ferritin, and Cu values were examined, respectively. The expression of ALP as protein was also investigated by using an enzymehistochemical and an immunohistochemical staining methods. Serum ALP values were elevated in 37.4% of RA (245.2 +/- 91.2 IU/L), and significantly higher than those of OA (192.3 +/- 45.2 IU/L: P < 0.01, RA v.s. OA). The serum CRP, and ferritin values each had a relation with the serum ALP activity. Fluid ALP concentration of RA was 110.3 +/- 40.1 IU/L, and that of OA, 83.6 +/- 15.0 IU/L (P < 0.05), respectively. In RA, a predominant isozyme was liver-type one both in the sera (91%) and the synovial fluid (59%). However, this result means that bone-type one was more abundant in the synovial fluids (41%) than those in the sera (9%). An enzymehistochemical and an immunohistochemical studies revealed that ALP was positive in a perivascular area, sublining cells, and a part of vascular endothelium in RA. In contrast, the synovial tissue from OA and a healthy patient exhibited only a weak staining. In RA, a positive correlation between the elevation of serum ALP and the disease activity was confirmed. Furthermore, we elucidated that ALP is produced in RA synovium.