Mycosis fungoides, which is characterized by a malignant infiltrate of T lymphocytes involving the epidermis, can be confused with other inflammatory skin diseases. We report the case of a patient with skin lesions containing an infiltrate of atypical lymphocytes with epidermotropism. This patient's condition was initially diagnosed as mycosis fungoides. Repeated biopsy samples had the histologic features of chronic cutaneous lupus erythematosus. The patient had a strongly positive antinuclear antibody response and the clinical lesions responded to hydroxychloroquine, however, and these findings led to an altered diagnosis. Other disorders that either clinically or histologically mimic mycosis fungoides are reviewed, and the diagnostic evaluation of patients in whom mycosis fungoides is suspected is summarized. Chronic cutaneous lupus erythematosus should be added to the list of diseases that can mimic mycosis fungoides.