True histiocytic lymphomas (THLs) are rare tumors in which the malignant cells show morphologic and immunophenotypic evidence of histiocytic differentiation. We describe THLs that arose after therapy for one case of T-lineage lymphoblastic lymphoma (LyL) and two cases of acute lymphoblastic leukemia (ALL) (both CD10+, one pre-B phenotype). The lymphoblastic neoplasms were not unusual in any way, and responded well to standard therapy. The THLs arose 10 to 20 months after complete remission was achieved for the lymphoblastic neoplasms, at which time there was still no clinical or pathologic evidence of the lymphoblastic neoplasms. All three THLs exhibited clinical and morphologic features of malignancy. Neoplastic cells in the THLs had abundant eosinophilic vacuolated cytoplasm and pleomorphic nuclei, and expressed histiocytic antigens in the absence of lymphocyte-specific lineage markers. Because THLs are rare neoplasms, their occurrence after otherwise successful therapy for lymphoblastic neoplasms in these three cases may constitute a distinct clinicopathologic entity.