The association of viral and Mycoplasma pneumoniae infections with acute exacerbations of chronic bronchitis was studied by serologic or isolation techniques in 46 adult men during the five years from 1964 through 1968. Serologic evidence of viral or M. pneumoniae infection was detected in 25% of 166 episodes of exacerbation and 14% of 138 remission periods (P = 0.02). Influenza A virus, parainfluenza virus type 3, and coronavirus OC43 predominated; infections with other viruses were infrequent. Infection with M. pneumoniae was detected serologically in four patients, but this organism was never isolated from sputum specimens. Rhinoviruses were isolated from frozen-stored sputum specimens in in 2.7% of the episodes of exacerbation and from 0.55% of the remission intervals (P not significant). These data suggest that although exacerbations of chronic bronchitis may be accompanied by viral and M. pneumoniae infections, patients with chronic bronchitis also acquire such infections without a worsening of their respiratory status.