Nasal biopsy findings in malignant histiocytosis presenting clinically as lethal midline granuloma are characterised by necrosis and infiltration of atypical histiocytic cells with a diffuse positive reaction for non-specific esterase. This cellular character was common to midline malignant reticulosis, and midline malignant reticulosis and malignant histiocytosis are thought to be the same disease. Patterns of histiocytic infiltration in the nasal lesions of 19 cases are reported in this paper. Polymorphic and monomorphic patterns were observed in 11 and four cases respectively, on the initial biopsy, but subsequently the infiltrates frequently became monomorphic on serial biopsy. The reverse was not observed. Surface marker and cytochemical studies showed the true histiocytic nature of the proliferating cells, and necropsy findings justified the diagnosis of malignant histiocytosis.