Background. Irinotecan-based chemotherapy is a standard backbone of therapy in patients with metastatic colorectal cancer (CRC) or gastric cancer (GC). However, there is still a paucity of information concerning the efficacy and safety of irinotecanbased regimens in elderly patients. Patients and Methods. Using the patient cohort (n 5 1,545) from the UGT1A1 genotype study, we compared the efficacy and safety between elderly and nonelderly patients with metastatic CRC (n 5 934) or GC (n 5 611) who received first-or second-line FOLFIRI (irinotecan, leucovorin, and 5-fluorouracil) chemotherapy. Results. Despite lower relative dose intensity in elderly patients, progression-free survival and overall survival were similar between elderly (age 70 years) and nonelderly (<70 years) patients in the CRC cohort (hazard ratio [HR], 1.117; 95%