Pulmonary hyalinosis occurred in Beagles exposed to radon daughters with uranium ore dust. The lesion was composed of alveolar cells distended with material positive for periodic acid-Schiff (PAS) and oil red O that ultrastructurally consisted of a whorled arrangement of lamellar membranes suggestive of a storage disease. The high incidence in exposed dogs and the ultrastructural appearance suggested the material originated endogenously as a degenerative response to injury.