This study evaluated the histopathological changes in lung tissue induced by formaldehyde vapors in mice. A group of 20 adult male mice was divided into two groups for this purpose. The mice in group (I) are the controls, whereas the mice in groups (II) were exposed to formaldehyde vapors inhalation in a toxic dose (0.5 part per thousand) everyday for two months in the students dissecting room of South Valley University's Faculty of Veterinary Medicine. Lung samples were prepared for light microscopic examination after exposure. Gross structural abnormalities in the lungs were identified, including congestion in most lobes and localized pneumonic organization. Thickening of the alveolar septum, epithelial hyperplasia in the bronchioles, proliferative capillaries, pulmonary vasculitis, hyperplastic lymphocytic aggregations in the parabronchiolar area, pulmonary fibrosis, and precancerous modifications were detected under light microscopy (metaplasia of goblet cells and dysplasia of the bronchiolar epithelium). It was found that inhaling formaldehyde has an irritating toxic effect in addition to its harmful carcinogenic effect on the lungs of mice, which is related to exposure durations.