The clinical and pathologic features of 86 roentgenographically solitary pulmonary granulomas were reviewed to determine etiology and to provide guidelines for histologic evaluation. Fungal or acid-fast organisms were identified within the tissue in 60 cases (70%) and fragments of a helminth were found in one. The organisms were almost always present in the center of necrotic granulomas, and examination of two blocks containing active granulomas was usually sufficient for their identification. Microbiological cultures were less productive than direct examination of the tissue. In 25 cases an infectious etiology could not be identified: two were diagnosed as hyalinizing granuloma, one as Wegener's granulomatosis, and 22 were not further classified. A prominent overlapping spectrum of histologic features was found between infectious granulomas and Wegener's granulomatosis, suggesting that the latter may represent an abnormal immune response to an infectious agent that is no longer identifiable within the tissue. Caution is urged in diagnosing limited Wegener's granulomatosis and other pulmonary angiitides in patients with roentgenographically solitary granulomas.