Background: Colorectal cancer (CRC) is the 3rd most common malignancy in Ireland with over 2700 cases annually. Approximately 20% are diagnosed with stage IV disease. The aim of this study is to evaluate the response to chemotherapy at primary and metastatic sites and review the frequency of intervention required to palliate the intact primary tumour in patients with stage 4 inoperable CRC in an Irish tertiary referral centre. Methods: A retrospective review of medical records was completed, identifying stage 4 CRC patients with primary tumour in situ diagnosed between January 2014 and December 2019, treated with chemotherapy (oxaliplatin or irinotecan based +/- bevacizumab or EGFR monoclonal antibody). Data and survival analysis were obtained using Kaplan-Meier methods. Results: 50 eligible patients were identified; 60% male, 40% female with a median age of 62 years. 2% had a transverse colonic primary, 32% right and 44% left sided and 22% had a rectal primary. 36% presented with liver metastasis only, 4% lung metastasis alone and 20% both. 48% were KRAS, 4% NRAS and 4% BRAF mutation positive while 1 patient was identified as having microsatellite instability. All patient received first-line chemotherapy either oxaliplatin or irinotecan based, 18% with the addition of Bevacizumab and 24% with EGFR monoclonal antibody. Overall response to first-line chemotherapy at the primary site and metastatic sites was assessed radiologically; 42% displaying a partial response, 36% had stable disease while 18% had progression at primary site. At the metastatic sites 50% responded, 10% stable disease and 40% progressed. Complication at primary tumour site included: obstruction 12%, with perforation in 6%, bleeding 10%, pain at tumour site in 6%, and one patient developed an abscess. Overall, after chemotherapy 76% of all patients did not require further intervention to manage primary site. 6% underwent curative surgery with resection of primary and metastatic lesions. Of those who had palliative intervention; 10% underwent palliative colostomy/ileostomy, 12% palliative radiotherapy, and 2% both. Overall survival was 14 months. At time of analysis 14% were alive, 10% receiving treatment and 4% on radiological surveillance. Conclusions: This retrospective study confirms that palliative chemotherapy +/- targeted therapy is effective in controlling the primary tumour in stage 4 inoperable CRC. In addition, it reveals a nearly 80% partial response or stable disease radiologically at the primary site after first-line chemotherapy. Furthermore, progression was significantly lower at primary site compared to distant metastasis (18% vs 40%). Almost 75% did not require palliative intervention for their primary tumour. Overall survival in our centre is higher compared to internationally observed data.