Recent investigations suggest an interdependence between blood fluidity and walking performance in patients with peripheral arterial occlusive disease (PAOD). Therefore, various blood fluidity variables (erythrocyte aggregation, erythrocyte flexibility, plasma viscosity, plasma proteins) were studied in groups of healthy subjects and claudicants with severe PAOD (exhausted perfusion reserve) receiving intravenous treatment with Trental (600 mg Pentoxifylline b.i.d., 21 days). Erythrocyte flexibility (expressed by filterability through micropore filters), red cell aggregation and plasma viscosity deteriorate with progression of disease especially in Stage IIb and III Fontaine classification, with walking distance below 150 m. Trental treatment resulted in patients with advanced POAD stages in improvement of red cell filterability, red cell aggregation, decrease of plasma viscosity, increase in absolute walking distance and relief from rest pain, suggesting that such patients are accessible to conservative treatment with hemorheologically active agents.