Background:Malignant histiocytosis is a rare neoplasm composed of abnormal histiocytes typically affecting the liver, spleen, lymph nodes, and bone marrow. This entity has been rarely documented involving the skin and has never been reported confined to the skin.Main observations:A 74-year-old white man presented to the dermatology clinic with complaints of a non-healing ulcerated lesion on his cheek of several months duration. Histopathological examination revealed a poorly circumscribed neoplasm consisting of pleomorphic epithelioid cells with abundant foamy cytoplasm. Immunohistochemistry was positive for CD-43, CD-68, and lysozyme, but negative for CD-3, CD-20, CD-30, CD-34, SMA, CD-1a or S-100. The prominent CD-68 and lysozyme staining along with the histological features, the clinical presentation of erythematous nodules with diffuse erythematous plaques, and absence of bone marrow findings, led to the diagnosis of malignant histiocytosis confined to the skin.Conclusion:Malignant histiocytosis involving the skin is rare. The presence of large pleomorphic epithelioid cells with foamy cytoplasm, with or without engulfed erythrocytes should alert the dermatopathologist to the possibilityofmalignanthistiocytosis.Appropriatimmunohistochemical evaluation, including CD-43, CD-68, CD-1a, S-100, and lysozyme, should be completed to confirm the diagnosis.