Objective: To show the importance of tissue Cytomegalovirus (CMV) PCR to diagnose CMV infection in patients with treatment-resistant ulcerative colitis. Methods: Fifteen patients who had previously been diagnosed as ulcerative colitis with clinical, endoscopic, histological, and radiological criteria and who had referred to our clinic with acute exacerbation who were diagnosed with CMV colitis and received CMV treatment, were evaluated retrospectively. After the demographic and clinical characteristics of the patients were noted, Mayo scores were evaluated to determine ulcerative colitis activity. Patients' age, gender, laboratory values, type of colitis, and Mayo scores were recorded. Biopsy results, CMV IgM, serum, and tissue CMV DNA levels were noted. Descriptive statistical parameters of the demographical data were calculated. Results: Eleven of the patients were male, and four were female. The mean age of the patients was 42±11,3. The mean duration of the disease was 28±46 months. Twelve patients had pancolitis, and three patients had distal colitis. All patients' Mayo score was 12 points. All patients were steroid-resistant, and none had previously received biological agent therapy. 7 of 15 patients had serum CMV DNA levels over 1000 copies/mL. Tissue CMV DNA levels of 8 patients were found higher than 250 copies/mg, although serum CMV DNA levels were below 1000 copies/mL. Pathology samples of 11 patients were evaluated, and CMV inclusion bodies were not detected. All patients received ganciclovir for CMV treatment. After treatment, CMV DNA of all patients was negative, and diarrhea and inflammation markers were reduced. Conclusion: For the diagnosis of CMV, CMV-PCR in colon tissue specimens should also be considered in addition to endoscopic appearance and serum CMV DNA levels.