BACKGROUND Apheresis is a procedure in which one of the components of blood is removed. The aim of therapeutic plasma exchange (TPE) is to remove a large fraction of the patient's plasma from the body, and to exchange this with replacement solutions using automatic devices. With this procedure circulating pathogens and toxins are reduced. Before each TPE results of a baseline basal complete blood count, serum protein electrophoresis, coagulation tests and serum electrolytes must be known. The efficacy of this therapy is assessed only by these values. The proteins responsible for disease may be monoclonal proteins, cryoglobulins, lipoproteins, auto or allo antibodies or toxins. In this study, we aimed to compare the effects of several replacement fluids on plasma viscosity and oncotic pressure. At the same time, we evaluated the correlation between plasma viscosity and oncotic pressure. METHODS 111 TPE were performed on 42 patients. Before TPE, the patients whose veins were not suitable were catheterised either by using a subclavian or jugular 11F dialysis catheter. At each session, approximately 1-1.5L of plasma was exchanged. The procedure was performed with albumin in patients whose albumin was under 3gr/dl. Over this value, the exchange fluids were randomised. RESULTS When the overall results were analysed, there was no statistically significant difference between groups 1 (HES+albumin) and group 3 (albumin). The statistical difference between group 2 and 3 was significant, but no difference was observed between group 1 and 2. According to the decreasing plasma viscosity, there was a significant difference between group 2 and group 3, but there was no difference between group 1 and group 2. CONCLUSIONS The replacement solutions used for plasmapheresis are similar when compared for hemorheologic effects, but we have chosen fresh frozen plasma because of fewer side effects.