Pneumonia is a major cause of death in the elderly. To investigate the role of silent aspiration in community-acquired pneumonia, we examined the occurrence of silent aspiration during sleep in 14 elderly patients with acute episode of pneumonia and 10 age-matched control subjects by a new technique using indium111 chloride. Scanning of the thorax demonstrated that 71% of patients aspirated, whereas aspiration was observed in only 10% of control subjects. The percentage of positive scans was significantly higher in patients with acute episode of pneumonia than in control subjects (p < 0.02). The results may indicate an important role of silent aspiration in the development of community-acquired pneumonia in the elderly.