We wanted to see whether the active inflammation in Behcet's disease (BD) can cause a thrombotic disorder by decreasing the protein S (PS) activity, and we evaluated the relationship between the decreased PS activity and the disease activity of BD. We included 122 patients with BD whose PS activity levels were measured. In 51 patients, the PS activity was measured again when there were changes in the number of items of "The Behcet's Disease Current Activity Form (BDCAF)". The thrombosis rate was 2.5% (3/122), and the PS activity was low in all three of the patients with thrombosis. The incidence of low PS activity in the total 122 BD patients was 27% (33/122). The incidence of the low PS activity in the active BD patients was 33.7% (31/92), and this was significantly more frequent than in the inactive BD patients, (6.7%, 2/30) (chi(2)-test, P value = 0.0038). The decrease of PS activity had good correlation with the increase of the number of BDCAF items (r = -0.351, P value = 0.012). The PS activity decrease is related to the BD activity. The low PS activity can be the risk factor for thrombotic disorder and also the activity marker for BD and other inflammatory diseases.