An elderly woman presented with anaemia, thrombocytopenia and multifocal lytic and sclerotic bone lesions. Trephine bone marrow biopsy demonstrated widespread involvement by systemic mastocytosis (SM). The neoplastic mast cells expressed mast cell tryptase, CD117, CD25 and CD9, and were accompanied by compact sheets of atypical large histiocytic cells, expressing CD68, CD4, S-100 protein and CD14, in keeping with a concomitant histiocytosis (SM-AHNMD). Mutation analysis revealed the activating point mutation D816V of the c-kit proto-oncogene in microdissected pooled bone marrow mast cells. Partial remission was achieved using interferon alpha. To the best of our knowledge this is the first reported case of SM-AHNMD with histiocytosis as the non-mast cell component.