We present a case demonstrating the diagnostic work-up of a patient undergoing azathioprine treatment for inflammatory bowel disease (IBD), diagnosed with an acute cytomegalovirus (CMV) infection and CMV colitis. An ¹⁸F-FDG positron emission tomography/computed tomography (PET/CT) performed 2 weeks after debut of symptoms revealed pathological ¹⁸F-FDG uptake in the left side of the colon mucosa, mimicked activity of IBD. However, a diagnosis of CMV colitis was based on the presence of CMV IgM antibodies, a seroconversion of CMV IgG antibodies, presence of CMV DNA in plasma and the finding af CMV DNA in biopsies from the intestinal mucosa. The patient responded to treatment with ganciclovir. This case highlights that a positive ¹⁸F-FDG PET/CT scan of the colon can be due to CMV colitis.