Various urodynamic studies have been used in patients with bladder outlet obstruction in order to evaluate the degree of obstruction, the results of therapy and postprostatectomy conditions. Radionuclide urodynamic study was performed in 27 patients with bladder outlet obstruction and 30 normal controls. The parameters evaluated were voiding time, 50% voiding time, average flow rate, peak flow rate, corrected peak flow rate, ejection fraction of the bladder and residual urine. Voiding time, 50% voiding time and residual urine of patients were significantly larger than controls and average flow rate, peak flow rate, peak corrected flow rate and ejection fraction were significantly lower in patients. This method was noninvasive procedure for determining of voiding parameters and it avoids the extraexamination needs to determine the residual urine.