We present a case of a patient with common variable immune deficiency presenting as the Letterer-Siwe syndrome (disseminated Langerhans cell histiocytosis). To our knowledge, this is the only known patient with this association. The clinical presentation was chronic diarrhea, weight loss, recurrent infections, hepatosplenomegaly, and interstitial pneumonitis. Laboratory evaluation revealed evidence of immunodeficiency, with agammaglobulinemia and diminished number and function of T cells. The diagnosis of Langerhans cell histiocytosis was confirmed by electron microscopic examination of the lung biopsy specimen demonstrating Birbeck granules in the cells of the infiltrate. It is known that patients with the combined immunodeficiency syndrome may present as disseminated Langerhans cell histiocytosis, and the case presented demonstrates that patients with common variable immune deficiency may similarly present. It is advisable that patients newly diagnosed with Langerhans cell histiocytosis be evaluated to screen for immunodeficiency. Conversely, patients presenting with combined immunodeficiency or common variable immune deficiency may display features of disseminated Langerhans cell histiocytosis. These associations must be considered in newly diagnosed immunodeficient patients.