Eight of the 9 affected horses had gross pathological, histopathological, and radiographic evidence of both systemic osteoporosis and pulmonary silicosis. Silicon and oxygen were the predominant elements found in crystals from lung tissue of 7 affected horses and tracheobronchial lymph node tissue from 9 affected horses; cytotoxic silica dioxide (SiO2) crystals (including quartz, cristobalite, and tridymite) were identified in both lung (6 affected horses) and tracheobronchial lymph nodes (8 affected horses).