Cmv is a life-threatening, opportunistic pathogen associated with significant morbidity and mortality in immunocompromised individuals. Citomegalovirus (cmv) retinitis is the most common ocular opportunistic complication and is a serious cause of vision loss in immunocompromised patients. Allergic drug reactions are unpredictable signs and symptoms that occur shortly (minutes to hours) after a particular drug is used at the appropriate dose. Allergic drug reactions can cause simple skin rashes or serious life-threatening anaphylactic or systemic reactions. However, desensitization is life-saving when there is no alternative medicine or when the patient's clinic is in question. Since our patient also had resistant cmv infection and we did not have an alternative drug, the drug had to be given by desensitizationto our knowledge, this is the first case of successful desensitization due to an early hypersensitivity reaction to foscarnet. Our result shows that desensitization to foscarnet is safe and effective in allergy.