Pathologic descriptions of Mycoplasma pneumoniae pneumonia in humans are scarce, and most descriptions are from autopsy studies. We studied open lung biopsy specimens from six patients with proved or probable M pneumoniae pneumonia and reviewed the findings on M pneumoniae pneumonia that have been reported in the literature, as well as the electron microscopic characteristics of the organism, to document the morphologic spectrum. Five of our six cases showed a typical but nonspecific polymorphonuclear leukocyte-rich exudate in the bronchiolar lumina, metaplastic cells that lined the bronchioles, a lymphoplasmacytic bronchiolar wall infiltrate, peribronchiolar septal widening, and adjacent hyperplasia of type II pneumocytes. In one patient, diffuse alveolar damage was superimposed on the above bronchiolar lesion, and the sixth patient had a nonspecific organizing pneumonia.