Silent aspiration would be major factor which predispose the bacterial infections in aged patients. Sixty-seven strains were isolated from culture positive cases. Anaerobes, S. pneumoniae, S. aureus, P. aeruginosa were the more frequently isolated strains of bacteria. Gram-negative rods were apparent in 39%, and Anaerobes were 19%, of 67 strains. Patients with diffuse panbronchiolitis are frequently affected by P. aeruginosa superinfection. The patients with longer duration, more severe lung function and more deteriorated roentgenological findings developed P. aeruginosa superinfection more easily. These infection in the lower respiratory tract significantly affect the prognosis of DPB patients. Using long-term administration of erythromycin against DPB, acute exacerbation were controlled in some patients and the frequency of their admission to hospital was lessened.