Background: Cisplatin (CDDP)-based chemotherapy is recognized as standard treatment for patients with advanced urothelial cancer (UC). In patients having lowered renal function, dosage of CDDP must be decreased for prevention of adverse events. However, relationship between reduction of CDDP and anti-cancer effects is not fully understood. The main aim is to clarify the influence of pathological anti-cancer effects by decreasing of CDDP in muscle invasive bladder cancer (MIBC). In addition, anti-cancer effects of radiation therapy in patients with decreased dosage of CDDP are also investigated. Methods: One hundred thirty three patients with MIBC selected CDDP-based intra-arterial chemotherapy. Biopsy was performed before intra-arterial chemotherapy, and then complete resection by trans-urethral resection was done after the therapy. Decreased dosage was defined as reduction ratio is over 20%, and 34 patients were judged as decreased dosage group. Two courses of chemotherapy with CDDP (70 mg/m²) and doxorubicin (30 mg/m² / epirubicin (30 mg/m²) were administered, and each course of chemotherapy was repeated approximately every 4 weeks. In 78 patients (58.6%), radiation therapy (total 40 – 60 Gy) began the day after chemotherapy administration. Results: The rate of down-stage after the chemotherapy in standard dosage group (63 of 99, 63.6%) was significantly higher (P=0.046) than that in decreased dosage one (15 of 35, 50.5%). In regard to pathological complete response (p-CR), patients with p-CR in decreased dosage group (n=9, 26.5%) were significantly lower (P=0.015) than those in standard dosage one (n=50, 50.5%). When similar analysis were performed in patients without radiation therapy, rate of p-CR in decreased dosage group was just 10.0%. On the other hand, p-CR was achieved in 33.3% in patients with radiotherapy of decreased dosage group. Conclusions: Reduction of CDDP may decrease its anti-cancer effects in patients with MIBC. On the other hand, combination of radiotherapy can increase the rate of p-CR in patients who treated with decreased dosage of CDDP. We believe our results are useful for discussion on treatment strategies in UC patients having lowered renal function.