Difficulties in the differential diagnosis of pulmonary hamartomas are illustrated on the basis of 19 patients with this condition, and with reference to the radiological literature. All the patients in the present series were treated surgically, either because a mistaken diagnosis of a bronchial carcinoma had been made, or because a carcinoma could not be excluded with sufficient certainty. In several cases the patient requested removal of the tumour in order to clarify the diagnosis after several years without symptoms. Pulmonary hamartomas do not always show the appearances described as typical; atypical appearances occur and were rather frequent in our material, amounting to 25%. Despite various difficulties, it is occasionally possible to put an hamartoma first in the differential diagnosis of a lung tumour, particularly if there are no clinical features to suggest malignancy. A definitive diagnosis can be made only by histology after removal of the tumour.