Activated neutrophils take a long time to pass through a narrow lumen like a micropore, and are supposed to play a deteriorating effect on microcirculation. Although the activation of neutrophils has been demonstrated in Behçet's disease, nobody analyzes the clinical activity of the disease by means of the rheological measure of neutrophils activity. Using a micropore (pore diameter 5 microns) filtration technique, we measured the filtration time of peripheral blood neutrophils, as a rheological measure of their activity, in order to determine the clinical activity of Behçet's disease. Twenty-one patients with Behçet's disease and 14 healthy control individuals were enrolled in the study. Symptoms and signs exhibited in the patients led us to distinguish the Behçet's disease into inactive and active cases. The latter were further differentiated into cases with absent symptoms and with present symptoms. Neutrophil filtration times were 11.5 +/- 4.8 s in the active cases with present symptoms, which were significantly (P < 0.05) larger than those (7.4 +/- 1.9 s) in the active cases with absent symptoms. The latter filtration times were further significantly (P < 0.001) larger than values (3.7 +/- 1.3 s) in the inactive cases and also those (4.8 +/- 1.2 s) in control subjects. Furthermore, increases in the filtration time obtained immediately after the exposure of cells to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP10 nM) were significantly (P < 0.01) larger in the active cases with present symptoms than those in the active cases with absent symptoms. The latter were also larger, but not significantly, than those in the inactive cases, and were significantly (P < 0.01) larger than those in control subjects. The present results demonstrate that the micropore filtration method reflects well the rheological activity of neutrophils as well as the clinical status of Behçet's disease. This method is much better than the measurement of O2 production to differentiate between active cases with absent symptoms and inactive patients or even control individuals. Furthermore, it is more sensitive and useful than laboratory data like the CRP value or the number of peripheral blood neutrophils.