Corynebacterium pseudodiphtheriticum, one cause of respiratory tract infection, is rarely considered as a pathogen because it is among normal bacterial flora colonizing the oral cavity. We report two cases of C. pseudodiphtheriticum causing respiratory tract infection--an exacerbation of chronic obstructive pulmonary disease and pneumonia--, both in the presence of underlying respiratory disease. Both required antibiotics after first-line chemotherapy failed. Isolates were susceptible to beta-lactams. Gram-staining of respiratory tract specimens is important in diagnosing this infection and determining appropriate antimicrobial use.