We report a case of squamous cell carcinoma of renal pelvis associated with giant hydronephrosis. A 71-year-old woman presented to our hospital with a complaint of abdominal fullness due to the right giant hydronephrosis. Although the diagnosis of her hydronephrosis was made about 20 years ago at another hospital, it had been left untreated. Computed tomography showed the right hydronephrosis of 20 x 20 x 25 cm in diameter and no evidence of tumor or calculus in the right urinary tract. For relief of her complaint, right nephrectomy was performed. The fluid content was bloody and 4,200 ml in volume. Histological examination revealed a flat type squamous cell carcinoma of the renal pelvis. This is the 30th case of renal pelvic malignant tumor associated with giant hydronephrosis reported in Japan. The literature was reviewed and the management of giant hydronephrosis was discussed.