Pleural adhesions in 155 young (3 to 15 montsh old) and 474 older sheep were examined macroscopically and by light microscopy. The pulmonary tissue under the adhesions was also examined. Two major types of adhesions were encountered, and were called type I and type II adhesions. Stages in the development and resolution of the type I adhesions were called fibrinous, early dense, dense and thin. Except for abnormalities of the blood circulatory system, the type I adhesions were similar to those in experimental models. Type II adhesions were proliferative lesions with villous projections, strands and nodules. Abnormality of the blood circulatory system also occurred in these adhesions. Most type I adhesions appeared to have developed in association with pulmonary abscessation and pneumonia. No relationship between type II adhesions and lesions in the underlying lung was obvious.