Background: A phase III study, JCOG1013, did not show the superiority of docetaxel plus cisplatin plus S-1 (DCS) to cisplatin plus S-1 (CS) in overall survival (OS) (Yamada Y, Lancet GH 2019). It is known that cisplatin and gimeracil (an inhibitor of dihydropyrimidine dehydrogenase contained in S-1) are excreted in urine. We previously reported (abstr 197, ASCO-GI 2021) exploratory analysis of JCOG1013 which showed creatinine clearance (CrCl) was associated with safety (Grade [G]4 hematological toxicity for CS, and G3-4 non-hematological toxicity for CS and DCS), but not with efficacy in either group. Here, we report the additional detail results of this exploratory analysis. Methods: Among 741 participants in JCOG1013, patients with serum creatinine level < 1.2 mg/dL were included in this analysis and categorized by CrCl and treatment into A1 (CrCl ≥ 80 mL/min, CS), A2 (60 < CrCl < 80, CS), A3 (CrCl < 60, CS), B1 (CrCl > 80, DCS), B2 (60 < CrCl < 80, DCS), and B3 (CrCl < 60, DCS). The dose (mg/m²) of C/S was 60/80 regardless renal function in group A (A1, A2 and A3), and that of D/C/S was adjusted in group B as follows: 40/60/80 in B1, 40/50/80 in B2, and 40/40/65 in B3. Adverse events, OS, progression-free survival (PFS), and objective response rate (ORR) were compared by CrCl in group A (A1 vs. A2 vs. A3) and group B (B1 vs B2 vs B3), respectively. Results: Of 723 pts (169/136/57 in A1/A2/A3 and 170/138/53 in B1/B2/B3), the median CrCl (mL/min) was 94.1/71.9/53.4 in A1/A2/A3 and 98.2/70.0/55.6 in B1/B2/B3. The relative dose intensity of C/S was 90.4/75.3%, 87.8/74.9% and 85.7/72.8% in A1, A2 and A3, and that of D/C/S was 87.5/77.7/74.9%, 85.8/61.2/72.7% and 87.8/49.4/58.3% in B1, B2 and B3. The incidence of G4 white blood cell decreased, G4 neutrophil count decreased, and G3-4 anorexia were 1.2/4.4/9.3% (P < 0.01), 4.8/11.1/18.5% (P < 0.01), 14.4/28.1/28.6% (P < 0.01) in A1/A2/A3, and 1.8/3.0/4.0% (P = 0.36), 27.3/24.8/20.0% (P = 0.28), 22.4/29.3/32.0% (P = 0.11) in B1/B2/B3, respectively. No significant association between CrCl and other adverse events was observed either in CS or in DCS group. The median OS was 15.4/15.5/15.4 months in A1/A2/A3 (P = 0.89) and 15.3/13.7/13.7 months in B1/B2/B3 (P = 0.72). The median PFS was comparable among A1/A2/A3 (7.1/6.8/6.2 months, P = 0.88) and B1/B2/B3 groups (7.5/7.2/7.8 months, P = 0.85). ORR showed no significant difference in A1/A2/A3 (58.9/57.8/46.9%, P = 0.31) and B1/B2/B3 groups (62.0/61.5/51.5%, P = 0.36). Conclusions: Dose modification according to renal function in the DCS arm could control the increase of severe toxicities, which were observed frequently in patients with low renal function in patients receiving fixed dose of CS. Clinical trial information: 000007652.