Systemic histiocytosis presenting as multiple sclerosis.

A patient resembling one with progressive multiple sclerosis in clinical presentation and by magnetic resonance imaging was studied in detail. Some features atypical for multiple sclerosis prompted a persistent search for an alternative cause. The diagnosis of a non-Langerhans systemic histiocytosis involving brain and bone was established and showed a partial response to radiation therapy. This patient illustrates the continued importance of a broad approach to the evaluation of possible multiple sclerosis, with particular attention to atypical features.