The urinary activities of alpha-naphthyl acetate esterases were measured for groups of bilharzial and nonbilharzial patients with benign urologic diseases and for others with bladder cancer. All these patients showed elevation in the urinary enzyme activity over that given by healthy controls. Bilharzial and nonbilharzial bladder cancer patients exhibited significant increase in urinary enzyme activity as compared with corresponding groups with benign urologic diseases A level of 50 units of enzyme activity was taken as a limit to discriminate between bladder cancer patients and those patients with benign urologic diseases. The specificity and sensitivity of this urinary test exceeded 90% with low falsely positive and negative results. The data of the present study recommended the use of urinary alpha-naphthyl acetate esterases activity as a preliminary screening test for bilharzial and nonbilharzial bladder cancer patients.