Background. In the absence of symptoms related to their primary tumor, patients with stage IV colorectal cancer can undergo medical treatment with their primary tumor in situ. In these patients, bowel obstruction is the most common primary tumor-related complication. We hypothesized that left-sided, circumferential, near-obstructing lesions and/or inability to advance the colonoscope beyond the primary tumor are associated with symptomatic bowel obstruction and are indications for prophylactic primary tumor resection (PTR) or colonic diversion. Patients and Methods. The medical oncology database of the University of Wisconsin Hospital was retrospectively reviewed. Inclusion criteria were presentation with stage IV colorectal cancer without previous treatment. Student's t-test and Fisher's exact test were used to compare continuous and noncontinuous variables, respectively. Results. Forty-nine patients met the inclusion criteria. None underwent colonic diversion or stenting during the course of their disease. At presentation, nine patients underwent PTR for obstructive symptoms. Forty percent of patients with high-risk colonoscopic lesions required PTR at presentation, compared with3% of patients without high-risk findings. No patients with highrisk colonoscopic findings and/or left-sided lesions who did not undergo PTR at presentation developed symptoms of obstruction during medical therapy. Conclusion. In stage IV colorectal cancer, circumferential, near-obstructing lesions and inability to advance the colonoscope beyond the primary tumor are common colonoscopic findings and are associated with obstructive symptoms at the time of diagnosis. Left-sided lesions and/or high-risk colonoscopic findings do not predict bowel obstruction during medical treatment and should not be indications for prophylactic PTR or colonic diversion in asymptomatic patients. ©AlphaMed Press.