Indeterminate cell histiocytosis successfully treated with narrowband UVB.

We present a 47-year-old man with a sudden eruption of more than 100 reddish-brown papules, which histologically exhibited a dense dermal proliferation of large mononuclear cells with vesicular nuclei and abundant pale cytoplasm. Electron microscopy and immunohistochemistry revealed findings consistent with indeterminate cell histiocytosis and the patient responded well to treatment with narrowband UVB therapy.