Interstitial mycosis fungoides (IMF) is a rare histopathologic variant of mycosis fungoides (MF) that may mimic other inflammatory dermatoses, mainly interstitial granuloma annulare, inflammatory morphea, and interstitial granulomatous dermatitis. Only small series and sporadic case reports of IMF have been described in the literature. We reviewed 27 specimens from 21 patients with IMF (M:F=11:10, median age 60) to better characterize clinical, histopathologic, and immunohistochemical features of this disease. Most patients presented clinically with patches and/or plaques. Conventional MF was documented before, concomitant with, or after IMF in 12 patients, whereas only in 2 patients different biopsies showed exclusive features of IMF over a period of 4 and 191 months, respectively. Histology revealed in all cases variably long, linear aggregates of dermal lymphocytes splaying the collagen fibers, involving predominantly the superficial and mid-dermis (6 cases) or the entire dermis (21 cases). Immunohistochemical stainings revealed a cytotoxic phenotype in 9/18 tested cases. Variable amounts of histiocytes/macrophages were found interstitially in all tested biopsies but never represented a population larger than that of T lymphocytes. Our study shows that IMF is a peculiar variant of MF with frequent cytotoxic phenotype. This histopathologic variant in most cases represents a transient pattern in otherwise conventional MF. Accurate clinicopathologic correlation and phenotypic studies of atypical dermal interstitial lymphohistiocytic infiltrates allow to make a correct diagnosis.