Hamartomas of the lung often present as asymptomatic, noncharacteristic masses that can seldom be differentiated from other lung masses such as primary cancer or metastases by conventional radiography. Transthoracic needle-aspiration biopsy (TNAB) has become a popular and reliable method for the diagnosis of a lung lesion, and it offers a valuable alternative to diagnostic thoracotomy. In our study, TNAB established the diagnosis of pulmonary hamartoma in 12 of 14 (86%) patients. In eight patients, one procedure (using one to three punctures) was sufficient to establish the diagnosis, and, in four patients, two procedures (using one to two punctures) were necessary. In two patients, the lesion was missed on second and/or third biopsy procedures, and the correct diagnosis was obtained at surgery. Cytologic examination of the material was diagnostic in five of the 14 patients. Tissue specimens were sent in 13/14 patients, and findings of histologic examination established the correct diagnosis in 11 of these patients. One of the 12 patients in whom hamartoma was correctly diagnosed by TNAB underwent resection of his lesion, and histologic examination confirmed the diagnosis. In the remaining 11 patients, the lesions have been stable on follow-up chest examinations.