BACKGROUND The aim of this work has been to assay determination error values for the clearance determined by single- and multi-sample method and to compare the error values obtained for both methods. METHODS The assay has been conducted by two independent methods (Monte Carlo simulation, total differential). RESULTS The results indicate that the relative root mean square error of the clearance determination by multisample method is practically constant above CI values 150 ml/min and amounts to ab +/-3-4%. For values below 150 ml/min the error increases and reaches +/-12% at CI=70 ml/min. The one sample method is less accurate and error increases from +/-(8-9)% 550 ml/min to +/- 25% at 70 ml/min. Moreover, for the multisample method the error estimated from total differential exceeds that obtained from Monte Carlo produced by factory of 3 to 4. For the one sample clearance determination both error estimates yield similar results. In both methods of clearance determination inaccurate pipetting makes a dominant contribution to the overall error. CONCLUSIONS Form monitoring of patients' condition the multisample method should be employed. The use of one sample method should be limited to screening tests and for obtaining additional information, supplementing the dynamic scintigraphic studies