Background: Distal protection devices are increasingly used to prevent emboliz ation during percutaneous coronary interventions(PCI) in saphenous vein grafts(S VG) and native coronary arteries(NV). During interventions with the Filterwire d evice we have observed reduced flow that is reversible following removal of the filter(filter no reflow, FNR), which might be erroneously interpreted as true no reflow and might be associated with reduced capture efficiency of the basket. M ethods: We analyzed the incidence of FNR in 58 patients(60 lesions) at high risk of embolization undergoing PCI of either a SVG or a NV using the Filterwire(Bos ton Scientific, Natick, MA). Qualitative and quantitative angiographic analysis was performed, and the volume of collected debris was estimated using a photogra phic technique. Results: In our population, about 1/3 of the cases showed FNR, w hich was associated with angiographically visible filling defects within the bas ket, indicating macroembolism. However some patients (especially those undergoing vein graft interventions) showed filling defects without FNR, and some others FNR without filling defects. Thus we tried to under stand the predictors of FNR: FNR was associated with higher amount of collected debris(36.97±42.98 mm3 vs. 11.31±18.47 mm3, p=0.005), was neither prevented by abciximab, nor predicted by high thrombotic burden, increasing stent volume or need for predilatation. When patient with and without angiographically evident m acroembolisation were separately analyzed, a linear correlation of FNR with the quantity of debris was only apparent in the macroembolization group. Conclusions : Interventionalists should be aware of the “Filter No Reflow”, a common but r eversible angiographic complication when the Filterwire device is used. Reduced flow seen during these procedures should be treated conservatively. Mechanical o bstruction of the filter, but also other mechanisms(pharmacologically active deb ris? platelet aggregates?) play a role in this phenomenon. Background: Distal protection devices are increasingly used to prevent emboliztion during percutaneous coronary interventions (PCI) in saphenous vein grafts (SVG) and native coronary arteries (NV). During interventions with the Filterwire d evice we have observed reduced flow that is reversible following removal of the filter (filter no reflow, FNR), which might be erroneously interpreted as true no reflow and might be associated with reduced capture efficiency of the basket. M ethods: We analyzed the incidence of FNR in 58 patients (60 lesions) at high risk of embolization undergoing PCI of either SVG or a NV using the Filterwire (Bos ton Scientific, Natick, MA). Qualitative and quantitative angiographic analysis performed performed, and the volume of collected debris was estimated using a photogra phic technique. : In our population, about 1/3 of the cases showed FNR, w hich was associated with angiographically visible filling defects within the bas ket, indicating macroembolism. How ever some patients (particularly those undergoing vein graft interventions) showed filling defects without FNR, and some others FNR without filling defects. FNR was associated with higher amount of collected debris (36.97 ± 42.98 mm3 vs. 11.31 ± 18.47 mm3, p = 0.005), was neither prevented by abciximab, nor predicted by high thrombotic burden, increasing stent volume or need for predilatation. When patient with and without angiographically evident acroembolisation separately transferred, a linear correlation of FNR with the quantity of debris was only apparent in the macroembolization group. Conclusions: Interventionalists should be aware of the “Filter No Reflow”, a common but r eversible angiographic complication when the Filterwire device is used. Reduced flow seen during these procedures should be treated conservatively. Mechanical o bstruction of the filter, but also other mechanisms (pharmacologically active deb ris? platelet a ggregates?) play a role in this phenomenon.