Bladder instillation of cytosine arabinoside (CA), 400 mg in 20 ml sterilized distilled water, was performed in 30 cases of bladder tumors. The instillation was done everyday to inpatients or 3 times a week to outpatients, for a total of 20 procedures. The total effective rate was 33%. Although the effectiveness of CA bladder instillation was less than that of mitomycin C and adriamycin, the combination of CA and these drugs might be advisable. Neither general nor local side effect was observed.