We prospectively evaluated the efficacy and safety of a low concentrate citrate lock versus heparin lock in permanent single lumen hemodialysis catheters. The frequency of clot formation, complete catheter occlusion, flow problems and the use of urokinase as well as catheter infection episodes were monitored during 1370 dialysis sessions in 19 patients, randomised in two study groups. There was a significantly higher number of dialysis sessions with clot formation in the citrate group but regarding the need for urokinase bolus or infusion, complete obstruction of the catheter or local infections, there were no statistically significant differences between groups. The higher incidence of clotting in the citrate locked catheters had no repercussion on dialysis efficiency, effective blood flow or on the use of thrombolytic therapy. We found that low concentrate citrate is as safe as heparin for long-term interdialytic anticoagulation of permanent single lumen hemodialysis catheters but is more efficient from a pharmaco-economic viewpoint.