Keishi-bukuryo-gan (Gui-Zhi-Fu-Ling-Wan) (KBG) is one of the prescriptions in Japanese traditional medicine for improving the "oketsu" syndrome, so-called blood stasis syndrome. "Oketsu" syndrome is an important pathological conception in Japanese traditional medicine and often accompanies cerebro-vascular disorders. Previously, we were able to reveal a deterioration of RBC (Red blood cell) deformability and viscoelasticity in patients with "oketsu" syndrome. The purpose of the present study was to evaluate whether KBG has an effect on RBC deformability in comparison with pentoxifylline (PXF). The subjects were 30 male patients with multiple lacunar infarctions. Eighteen patients (44-79 yrs, mean +/- SD, 66.1 +/- 10.7 yrs) were treated with 12 g of KBG daily for 4 weeks (KBG group). Twelve patients (59-78 yrs, 70.7 +/- 6.4 yrs) were treated with 300 mg of PXF daily for 4 weeks (PXF group). Based on the "oketsu" score, the patients of each group were divided into two subgroups, a non-"oketsu" group ("oketsu" score 20 points or less) and an "oketsu" group ("oketsu" score 21 points or higher). KBG had significant effects on RBC deformability as evaluated by filtration method. KBG also significantly increased intracellular ATP content, as did PXF. Moreover, KBG was more effective for patients with a more severe "oketsu" state. However, PXF was effective only in patients with "oketsu" syndrome, who might have deteriorated RBC deformability. In conclusion, the effect of KBG on RBC deformability was by no means inferior to PXF.