Hamartomas are tumor-like masses comprising disorganized normal tissue elements. To
date, spontaneous hamartomas have been reported in several organs and tissues in rodents
but not in the lungs. Here, we report the first case of a hamartoma in the lungs of a
108-week-old female Wistar Hannover rat. Grossly, a white spot, 7 mm in diameter, was
observed on the costal surface of the left lung. Histopathologically, the nodular lesions
adjacent to the bronchioles comprised mature smooth muscle cells. The lesion was not
encapsulated and spread along the alveolar walls and ducts without compression of the
surrounding tissue. In the nodules, elastic fibers enclosed small lumens lined with factor
VIII-related antigen-positive endothelial cells. This structure suggested that the nodule
mimicked an artery. Moreover, structural abnormalities were observed within the
bronchioles and arterioles owing to the increased number of smooth muscle cells in the
surrounding tissues. These features suggested that this was a case of tissue malformation
rather than a neoplasm, leading to the diagnosis of a smooth muscle hamartoma of the
lung.