Indeterminate cell histiocytosis (ICH) is a rare proliferative disorder, in which the predominant cells share morphologic and immunophenotypic features from both Langerhans and non-Langerhans cell histiocytosis. We describe a 62-year-old man presenting a 2-month history of firm nodular lesions on the upper lip. Histopathology, immunohistochemical, and ultrastructural analysis showed typical findings of ICH. The patient was treated with thalidomide and almost complete regression of the lesions was reached within 7 months. Nevertheless, one month after remission, he developed an acute myeloid leukemia of the subtype monocytic leukemia (M5). The patient's condition rapidly worsened and he died due to a respiratory failure four weeks later. We present this case because apart of being rare it joins the effectiveness of thalidomide and the association with an acute monocytic leukemia. A review of the literature is made.