Acute retinal necrosis (ARN) is a clinical syndrome featuring severe vitritis and occlusive vasculitis characterised by full thickness necrotising retinitis. ARN is usually caused by an acute infection by either varicella zoster virus or herpes simplex virus, rarely cytomegalovirus (CMV). ARN often occurs in healthy adults; occasionally affecting immunocompromised patients with poor prognosis including significant visual loss and detachment of the atrophic retina regardless of antiviral treatment. We presented a man in his early 30s with a history of left eye floaters and blurred vision. He was diagnosed with T-cell acute lymphoblastic leukaemia 1 year ago and treated with chemotherapy and allogenic haematopoietic stem cell transplant 5 months ago. His clinical diagnosis was left eye ARN caused by acute viral infection with CMV being the most likely cause, which is rarely seen in immunocompromised patients. Our case highlighted a diagnostic and therapeutic challenge in the absence of guideline or evidence-based literature to follow.