Background: The eligibility criteria of renal function in cisplatin (CDDP) -based chemotherapy for urothelial carcinoma has generally been a creatinine clearance (CCr) ≥ 60 (mL/min) without adjusting for body surface area (BSA). While CDDP dosage is determined by body surface area, the use of CCr (mL/min) without correction for body surface area as a criterion for cisplatin eligibility may lead to CDDP overdose, especially in obese patients with borderline renal function. CCr with BSA modification (CCr mL/min/1.73 ㎡) may be more appropriate than without BSA modification for assessing renal function in patients with nonstandard body mass. Methods: This retrospective analysis included 82 patients who received CDDP-based chemotherapy as primary systemic treatment for urothelial carcinoma at Nagoya University in the past 10 years. For all patients, CDDP eligibility was determined by CCr mL/min by 24-hour urine collection, and the inclusion criteria for this study were CCr < 80 mL/min. To identify the possibility of a gap between CCr mL/min and CCr mL/min/1.73 ㎡ resulting in cisplatin overdose unit organ function, ΔCCr (difference between CCr mL/min and CCr mL/min/1.73 ㎡) and CDDP/CCr (actual dose of CDDP mg per unit CCr mL/min) were calculated and compared between the obese and standard-body groups. In addition, we compared the incidence of CTCAE Grade ≥3 adverse events (AEs) and the objective response rate (ORR) to chemotherapy in the two groups. In accordance with Japanese standards, BMI ≥ 25 was defined as obese patients. Results: There were no significant differences in patient background between the obese and standard groups in terms of age, gender, cancer type and status, and CDDP dose per unit BSA. CCr mL/min/1.73m2 was similar between the two groups, whereas CCr mL/min was significantly higher in the obese group, which resulted in a significantly higher ΔCCr in the obese group. In addition, CDDP/CCr was also greater in the obese group. Conclusions: The results of the current study suggest that the application of CCr mL/min to the assessment of renal function for CDDP eligibility may lead to CDDP overdosing, especially in obese patients with borderline renal function. Further larger studies are warranted to clarify whether or BSA correction should be applied to assess renal function for CDDP eligibility, since the method for assessing renal function may affect the occurrence of AEs and the efficacy of chemotherapy.