Objective The purpose of this study was to analyse the effectiveness and safety of first-line treatment of metastatic colorectal cancer (CRCm) in older patients treated in a tertiary hospital. Material and methods This was an observational and retrospective study, including patients aged 75 years or older, with CRCm, who received chemotherapy treatment in 2017. The main variables studied were type of treatment, Progression-Free Survival (PFS), Overall Survival (OS), dose reductions, and treatment delays due to adverse events. Results A total of 59 patients (71.2% men) with a median age of 76 years were enrolled in this study. About 70% presented colon cancer, with the left colon being the most frequent location. They were treated with 9 different schemes, in most cases using polychemotherapy and biological agents. The median PFS and OS was 12 and 30 months, respectively. A total of 23/59 of patients started treatment at doses lower than recommended in the clinical practice guidelines. In terms of safety, 34/59 of patients had at least one dose reduction, and 30/59 suffered one treatment delay. The most frequent adverse reactions were asthenia, peripheral neuropathy, diarrhoea, and palmoplantar erythrodysesthesia. Conclusion Our patients presented baseline clinical characteristics similar to the general adult population, with no tumour characteristics associated with advanced age. The efficacy and toxicity were similar to those in the clinical trials, although our patients had more dose reductions. Considering the heterogeneity of patients and in the absence of clinical trials in the older population, real-life studies can be very useful.