Eczema herpeticum (EH), also known as Kaposi's varicelliform eruption, is characterized by disseminated papulovesicular eruption caused by a herpes simplex virus infection superimposed on a pre-existing dermatosis. The eruption usually occurs in individuals with atopic dermatitis or other pre-existing dermatosis such as Darier's disease, pemphigus foliaceus, and ichthyosis vulgaris. Mycosis fungoides (MF) is the most frequent manifestation of cutaneous T cell lymphoma but its cause and pathophysiology remain unclear. Herpes virus infection occurs not uncommonly in MF patients. However, only a few MF patients with EH have been described. A 58-year-old male presented with erythematous patches and plaques on the face. Two years ago, he was diagnosed with MF stage IIA and he had been receiving treatment including oral retinoid and ultraviolet A-1 therapy. Because the patient suffered from severe pruritus and his lesions had worsened, he was started to treat with topical imiquimod 5% cream 3 times weekly. After applying topical imiquimod 12 times, painful edematous plaques developed on his face. After 3 days, grouped vesicles and erosions developed in the erythematous areas of the face. Tzanck’s smear from a vesicle on his face showed multinucleated giant cells. The diagnosis of EH was thus made. The patient was treated with valacyclovir (3,000 mg/day) for 7 days and the lesion was completely resolved. Herein, we report a case of EH in MF.