Patients with autoimmune lymphoproliferative syndrome (ALPS) have defective lymphocyte apoptosis with increased risk for lymphoid malignancies. Herein, we report a patient with ALPS who developed histiocytic sarcoma in a background of sinus histiocytosis and massive lymphadenopathy or Rosai- Dorfman disease. This patient had documented ALPS type Ia with a germline missense mutation in exon 9 of the TNFRSF6 gene (973 A>T, D244V) encoding Fas (CD95/Apo-1). He presented at 10 months with hepatosplenomegaly and autoimmune hemolytic anemia and was diagnosed with ALPS. At the age of 6 (1/2) years, he developed classic Hodgkin lymphoma which was treated using standard chemotherapy. Two years later, a biopsy of a positron emission tomography-positive axillary node showed features of ALPS and focal involvement by sinus histiocytosis and massive lymphadenopathy. Thereafter, the patient continued to have continued lymphadenopathy and progressive splenomegaly, leading to exploratory surgery at the age of 13 years for suspicion of lymphoma. Para-abdominal nodes revealed sheets of malignant- looking histiocytes with increased mitotic activity and areas of necrosis, indicative of histiocytic sarcoma. Spleen and lymph nodes also showed involvement by Rosai-Dorfman disease. Both components had an identical phenotype of S-100+/CD68+/ CD163+. The occurrence of malignancies involving 2 separate hematopoietic lineages in ALPS has not been reported earlier. Given the central role of defective Fas signaling in ALPS, histiocytes may be yet another lineage at risk for neoplastic transformation secondary to a block in apoptosis.