Background: There is no consensus regarding resection of the primary tumour with few or absent symptoms in patients with synchronous unresectable metastatic colorectal cancer. A potential benefit of resection of the primary tumour is to prevent complications of the primary tumour in later stages of the disease. Retrospective studies also show a potential survival benefit for patients undergoing resection. However, surgery can cause severe morbidity and mortality in this patient group. We hereby propose a randomized trial in order to demonstrate that resection of the primary tumour improves overall survival. Methods: The CAIRO4 study is a multicentre, randomized, phase III study of the Dutch Colorectal Cancer Group (DCCG). Patients with synchronous unresectable metastases of colorectal cancer and few or absent symptoms of the primary tumour are randomized 1:1 between systemic therapy only, and resection of the primary tumour followed by systemic therapy. Patients will be stratified according to location of the primary tumour (colon versus rectum), WHO performance status (0-1 versus 2), hospital of inclusion, serum LDH (normal versus abnormal) and number of metastatic sites (single versus multiple). To demonstrate a survival benefit of 6 months in the experimental arm, a total of 218 events are needed (80% power, significance lever 0.05). Accounting for accrual time and follow-up, we need a total of 360 patients (180 patients per arm). Systemic therapy will consist of fluoropyrimidine-based chemotherapy in combination with bevacizumab. The primary objective of this study is to determine the clinical benefit in terms of overall survival of initial resection of the primary tumour. Secondary endpoints include progression free survival, surgical morbidity, quality of life and the number of patients requiring resection of the primary tumour in the control arm. Accrual has started in September 2012. As of January 2015, 64 centres in the Netherlands and Denmark are participating in the CAIRO4 study. Clinical trial information: NCT01606098