Purpose: We report five cases of cytomegalovirus (CMV) retinitis in immunocompetent patients with diabetes mellitus (DM).Methods: This was a retrospective observational case series of five immunocompetent patients with CMV retinitis. All patients had DM and variable degrees of DM retinopathy. After discovery of the retinitis, we assessed the patients in terms of ophthalmologic history, systemic conditions, underlying disease, and the medications used for treatment, such as immunosuppressants. This was followed by blood and immunology tests. None of the five patients were immunocompromised. We confirmed the diagnosis of CMV retinitis by polymerase chain reaction (PCR) analysis; samples were collected by anterior chamber paracentesis. All patients were treated with intravitreal antiviral injection until negative results were obtained for the anterior chamber PCR.Results: All five immunocompetent patients who had developed CMV retinitis, had a history of diabetes of at least 15 years. The average duration of diabetes was 17.4 ± 2.4 (15-20) years, the average HbA1c was 7.3 ± 0.5% (6.8-8.0%), and the average duration and numbers of Intravitreal antiviral injections were 23.8 ± 8.5 months (14-36 months) and 14.8 ± 3.2 times (10-18 times), respectively.Conclusions: We report this case series because CMV retinitis can be complicated even in diabetic patients shown to be immunocompetent.