In the past six years, at the Mayo Clinic, 17 hamartomas of the lung have been removed surgically and the diagnosis has been made subsequently by microscopic examination of the tissue. Since, so far as we know, this is the largest series of these tumors to have been observed clinically and surgically, we believed that an attempt should be made to determine whether diagnosis could be made prior to surgical removal of the lesion. We felt particularly responsible for determining what roentgenography could contribute to such diagnosis. The microscopic sections and roentgenograms representing each of the 17 cases were reviewed. Partial Survey of Reported Experience Definition and Classification: The term “hamartoma,” from the Greek word meaning “to err,” was originated by Albrecht (1) in 1904. His definition, in translation, is “tumor-like malformations in which in truth one can demonstrate only an abnormal mixture of the normal developmental components of the organ in which they occur, whether it be with regard to the quantity, to the arrangement, or to the degree of development, or in all three respects. The conclusion that can be drawn from such a histologic analysis of these structures is evident; it should be assumed that their origin likewise results from such an abnormal mixture or from disturbances of their development.” Albrecht used this term to explain cavernomas of the liver, cavernomas of the spleen, and tubular adenomas of the liver, and suggested its application to a great many benign tumors. At the present time, the designation “hamartoma” is applied to the benign tumors of the bronchus which are the subject of this paper and, in addition, to specific benign tumors in other organs. Prior to Albrecht's work, and before the term was more widely accepted, hamartomas of the lung were frequently described as chondromas (2–4). Jaeger (5) used the term “hamartochondroma” to specify the dominant element. In 1938, Womack and Graham (6) included chondromas of the lung under the general designation “mixed tumor of the lung.” Cid (7), writing in 1940 of pulmonary hamartochondromas, considered chondroma to be one of the choristoblastomas and classified adenomas and hamartochondromas as varieties of hamartoma. McDonald (8), however, has not included pulmonary adenomas among the hamartomas. In 1948, Hall (9) contributed reports of 4 new cases of pulmonary hamartoma and stated that the total number of reported cases of chondroma and hamartoma was more than 100. With the addition of 14 of the 17 cases described herein (3 of the 17 cases having been previously reported by McDonald, Harrington, and Clagett, 10) the total is probably in the neighborhood of 120 or 125. Situation: The distribution of the lesions in the lung demonstrates no clear predilection for either side nor for upper or lower lobes (4, 10). The tumors usually are just subpleural, although they have been found in nearly every possible relation to the pleura.