A method to assess heparin kinetics and individualize dosages was examined in 27 patients during chronic hemodialysis. Pretreatment heparin sensitivities were determined to establish the relationship between heparin concentration and activated clotting times (ACTs). Distribution volume was calculated by dividing the heparin loading dose by the 5-min heparin concentration. Assessments were made during three different dialysis periods. There was a 10-fold range in pretreatment heparin sensitivities. Intrapatient heparin sensitivity remained relatively consistent between dialysis periods. The degree of heparin sensitivity was significantly correlated to baseline ACT. Patients post-splenectomy were more sensitive to heparin. Large intra- and interpatient variation in distribution volume was also observed. Men, patients, <60 years old, and patients post-splenectomy represented variables that contributed to higher heparin dose requirements.