A prospective, randomized and open study was conducted to evaluate the ability of an individualized heparin dosing protocol to achieve and maintain whole blood activated partial thromboplastin times (WBAPPT) within a predetermined range in hemodialysis patients. Thirty-one hemodialysis outpatients who received a total of 99 dialyses were studied. Systemic heparinization was achieved with a loading dose and a continuous infusion. WBAPPT, used as a monitoring parameter for heparin therapy, was measured at 5 minutes and 1, 2, 3 and 3 1/2 hours after the start of heparinization. An initial heparin loading dose of 50 u/kg dry weight was used, with a continuous infusion of 15 u/kg/h and a sliding scale to adjust infusion rates to target WBAPPT in the goal range of 150-190 seconds. Individual heparin loading doses for subsequent treatment sessions were titrated to achieve the lowest dose which would still result in clear dialyzers at the end of hemodialysis. There was a high degree of interindividual variability in heparin requirements. A loading dose of 20-25 u/kg placed the majority (72.4%) of the patients within the 150-190 seconds range at the 5 min WBAPPT measurement. The sliding scale method was effective in keeping about 60% of the WBAPPT within the desired range throughout the rest of the hemodialysis session. As the loading or total doses were increased, the incidence of clear dialyzers increased. In conclusion, this heparin protocol is effective for individualizing heparin doses to achieve therapeutic WBAPPT in hemodialysis patients.