We studied 96 patients with superficial tumors of the urinary bladder treated by transurethral resection in order to investigate the value of IntravesicalCorynebacterium parvum(CP) to prevent recurrences. In 52 cases, two vials (2 ml, 4 mg CP) diluted in 60 ml buffer saline were intravesically administered weekly 12 times and then monthly for a period of 2 years. All patients were studied in a 3 year follow-up by cytology, histology and endoscopy. Recurrences were observed in 21.1% of cases in the CP treated group and in 54.5% of cases in the untreated group. CP immunostimulation is less effective in preventing recurrences in patients with malignant cells as indicated by urine smears in the post-operative period. Morphological changes of the bladder wall due to CP administration in patients without recurrences are described. Chronic lymphocyte infiltrate appears to be an essential event for the action of CP as an adjuvant therapy in urinary bladder cancer.