Subtle presentation and the frequent lack of on-site physicians complicate the diagnosis of pneumonia in nursing home residents. We sought to identify clinical findings (signs, symptoms, and simple laboratory studies) associated with radiographic pneumonia in sick nursing home residents. This was a prospective cohort study. The residents of 36 nursing homes in central Missouri and the St. Louis area with signs or symptoms suggesting a lower respiratory infection were included. We compared evaluation findings by project nurses with findings reported from chest radiographs. Among 2334 episodes of illness in 1474 nursing home residents, 45% of the radiograph reports suggested pneumonia (possible=12%; probable or definite = 33%). In 80% of pneumonia episodes, subjects had 3 or fewer respiratory or general symptoms. Eight variables were significant independent predictors of pneumonia (increased pulse, respiratory rate =30, temperature =38 degrees C, somnolence or decreased alertness, presence of acute confusion, lung crackles on auscultation, absence of wheezes, and increased white blood count). A simple score (range = -1 to 8) on the basis of these variables identified 33% of subjects (score > or =3) with more than 50% probability of pneumonia and an additional 24% (score of 2) with 44% probability of pneumonia. Pneumonia in nursing home residents is usually associated with few symptoms. Nonetheless, a simple clinical prediction rule can identify residents at very high risk of pneumonia. If validated in other studies, physicians could consider treating such residents without obtaining a chest radiograph.