Mass chest X-ray surveys have revealed that many intrathoracic lesions other than tuberculosis may be found in apparently healthy people. Particularly important are the isolated well defined nodules which may offer considerable difficulty in diagnosis and treatment. Such asymptomatic nodules may represent an early phase in the development of bronchogenic carcinoma. In a group of 25 silent isolated nodules observed by the author, 6 were found to represent malignant lesions. Unfortunately, in the stage in which circumscribed nodules have often been detected exact diagnosis has rarely been possible. There are no satisfactory clinical or roentgenologic criteria for a specific differentiation of the variety of lesions which may appear as isolated nodules or which can exclude the presence of a malignant growth. It is, therefore, emphasized that with few exceptions, asymptomatic isolated intrathoracic nodules are correctly treated only with prompt surgical exploration if an exact preoperative diagnosis is impossible, or if the presence of a malignant tumor cannot be excluded with certainty. Summaries of representative case reports illustrate these problems.