In recent years, studies of the diagnosis and treatment of bladder tumors have remarkably advanced. But as the treatment in so-called “terminal stage”, there is only palliative therapy (urinary diversion) in order to maintain the renal function and reduce the vesical symptom. So, first, the experimental induction of bladder tumors was attempted as for 50 mongrel dogs by the method of oral administration of β-naphthylamine. As a result, benign and malignant tumors appeared in 24 bladders between 11 months and 42 months. Second, a study was made on the effect of urinary diversion upon the experimentally produced bladder tumors.The urinary diversion group comprised 12 cases (which had been dead within 3 months), 7 cases (between 3 and 6 months) and 3 cases (beyond 6 months); the control group, 1 case (within 3 months), 4 cases (between 3 and 6 months) and 2 cases (beyond 6 months). The preoperative findings was compared with postoperative ones of the urinary diversion and control groups from macroscopic and microscopic examinations. On stage: regarding the urinary diversion group, it was found that all showed a malignant tendency except one case, and the control group was the same. On grade: regarding the urinary diversion group, there were no changes in the malignant tendency in 3 cases, slight changes in 6, and remarkable changes in 6, and regarding the control group, there were no changes in 1 case and moderate changes in 6. On macroscopic findings there was no evidence of regression.It is concluded that diversion of the urinary stream does not inhibit the growth of β-naphthylamineinduced tumors of the bladder in dogs. But, from a clinical standpoint, it is considered that urinary diversion should be done in order to reduce vesical symptoms which are seen in case of inoperable bladder tumors.