Background: Actinic reticuloid is a severe persistent photodermatitis that usually affects older men. It is characterized by extreme photosensitivity to a broad spectrum of ultraviolet radiation. Actinic reticuloid has many of the clinical and histological features of mycosis fungoides and Sezary syndrome. No Korean studies are available regarding a histopathological and immunohistochemical comparison of actinic reticuloid and mycosis fungoides. Objective: This study was designed to evaluate the histopathological characteristics and conduct a immunophenotypic analysis to distinguish actinic reticuloid from mycosis fungoides. Methods: We reviewed the histomorphological findings of 10 patients diagnosed with actinic reticuloid and those of 15 patients diagnosed with mycosis fungoides from 1996 to 2012 at our clinic. In addition, an immunophenotypic analysis was performed on the skin to assess the proportions of CD4＋ and CD8＋ T cells in all patients. Results: We found a broad overlap in non-specific inflammatory histological features and more prominent histological features between the two diseases. Irregular acanthosis, vertically-streaked collagen in the papillary dermis, thickened and increased blood vessels, and an eosinophil and plasma cell infiltrate were significantly higher in patients with actinic reticuloid than in those with mycosis fungoides. Stuffed lymphocytes in dermal papillae and a psoriasiform lichenoid pattern were observed significantly more often in patients with mycosis fungoides. Solar elastosis (80%) was seen only in the actinic reticuloid cases. CD8-positive T cells were predominant in the epidermis in eight of 10 patients with actinic reticuloid. In contrast, CD4-positive T cells in the epidermis and dermis predominated in the majority of patients with mycosis fungoides (73.3%) compared with CD8-positive T cells. Conclusion: Histopathological findings and immunophenotypic analyses may be an important adjunct to distinguish actinic reticuloid from mycosis fungoides, but a correlation with clinicopathological findings, phototest, patch test, and photopatchtest is necessary to reliably distinguish actinic reticuloid from mycosis fungoides. Further prospective studies should be conducted on a larger number of cases regarding the differences between actinic reticuloid and mycosis fungoides.