Generalized eruptive histiocytosis, described in 1963 by Winklemann and Muller, is a reactive, self-healing form of non-Langerhans histiocytosis. Rare cases of atypical generalized eruptive histiocytosis have been reported in patients with hematopoietic malignancy, but the biological relationship between the two disorders is not known. We report an 84-year-old man with chronic myelomonocytic leukemia who presented with coalescing erythematous papules and plaques on the posterior neck, ear and lower lip, followed by development of blast crisis. Skin biopsy revealed a thick band-like dermal infiltrate of cells that exhibited morphologic features of macrophages or histiocytes and prominent elastolytic phagocytosis. These cells demonstrated a mature immunophenotype, expressing CD14 and CD68, with partial expression of CD13 but not CD1a, CD43, CD56, CD123, Langerin, or S-100 protein. Karyotype and fluorescence in situ hybridization analyses showed loss of the Y chromosome in bone marrow and skin specimens, providing evidence of a clonal relationship between the cutaneous eruption and the underlying chronic myelomonocytic leukemia. The presence of the same clone in skin and bone marrow specimens from our patient supports the possibility that atypical generalized eruptive histiocytosis is a marker for underlying hematopoietic malignancy. Discovery of additional cases may shed further light on the pathogenesis of this rare entity.