Ultrastructure in the differential diagnosis of pulmonary histiocytosis and pneumocystosis.

A child with histiocytosis X initially diagnosed at the age of 4 months had a series of pulmonary biopsies later in the course of the disease. The specimens were examined by light and electron microscopy. The primary disease was complicated by Pneumocystis carinii pneumonia. This study demonstrates the limitation of light microscopy and the necessity of ultrastructural examination to verify the simultaneous occurrence of an infiltrative disorder such as histiocytosis X in the presence of an infection.