The purpose of this study is to evaluate if the prolongation of activated partial thromboplastin time (aPTT/seconds) adjusted > 1.5 times the control value is a good schedule for heparin monitoring therapy. The authors have constructed a reference curve for the prolongation evaluation of heparin plasma concentration (units/ml) from corresponding aPTT/seconds. In this way, 6 patients at end-dialysis treatment showed that aPTT/seconds prolonged > 1.5 times the control value is equivalent to heparin plasma concentration lungh than the limit of safe therapeutic regimen. This methodology of investigation is easy to perform and allows a continuous monitoring of heparin therapy.