Trifluridine (FTD) / tipiracil hydrochloride (TPI) has been approved for the treatment of colorectal cancer, but its dose-limiting toxicity is neutropenia. Elderly patients are at high risk of febrile neutropenia and should be monitored carefully when administering FTD / TPI. In this study, we retrospectively investigated the incidence of adverse events, dose reduction, and number of administered courses in 8 old-old patients treated with FTD / TPI for colorectal cancer. The results showed that severe neutropenia of Grade 3 or higher occurred in 5 of the 8 patients. Six patients had a dose reduction, but 5 of them were able to continue treatment for at least 3 courses. The frequency and severity of adverse events, including neutropenia, appears to be higher in old-old patients, so it is important to take appropriate measures.