Two cases of bladder tumor producing granulocyte colony-stimulating factor (G-CSF) are reported. The first case was in a 79-year-old female patient. A large bladder tumor was diagnosed as right-sided hydronephrosis. The tumor consisted mostly of squamous cell carcinoma with a few transitional cells. Total cystectomy could not be performed because of direct invasion by the tumor into the pelvis. The patient died without aggressive treatment about 7 months after admission. Her leukocyte count consistently increased up to 76,200/mm3. The serum G-CSF levels were not analyzed. However, immunohistochemical examination revealed a high concentration of G-CSF in the tumor specimen. The other case was in an 80-year-old male patient. The patient, who had refused total cystectomy for bladder tumor (transitional cell carcinoma, grade 2, T2N0M0) 2 years earlier, underwent ureterocutaneostomy for obstructive renal insufficiency. Total cystectomy was not performed at this admission because of tumor invasion into the rectum and his advanced age. Radiotherapy was administered. However, he developed ileus caused by direct tumor invasion into the ileum. He died about 10 months after the urinary diversion. Leukocytosis, which improved transiently following radiotherapy, became more severe. The maximum leukocyte count was 49,500/mm3 just before death. The serum G-CSF levels during and after radiotherapy were 54 pg/ml and < 30 pg/ml, respectively. Immunohistochemical examination revealed the presence of G-CSF in the tumor. These findings suggest the production of G-CSF by the bladder tumor.