The American Thoracic Society (ATS) published guidelines for the treatment and management of community-acquired pneumonia in 2001, but the impact of adherence on outcomes such as mortality and length of stay is not well defined. A study of 780 patients with community-acquired pneumonia consecutively admitted to hospital over 1 yr was carried out. Nursing home patients were excluded. Overall adherence to antibiotics recommended in the ATS guidelines was 84%. The lowest adherence was found in patients admitted to an intensive care unit (52%), especially those at risk of infection with Pseudomonas aeruginosa (ATS group IVb). However, very few patients from this group were indeed infected with P. aeruginosa. This could be explained by the exclusion of the nursing home patients. There was a difference in mortality between patients that received adherent and nonadherent regimens (3 versus 10.6%). There was a difference in length of stay between patients receiving adherent and nonadherent regimens (7.6 versus 10.4 days). This result was confirmed on multivariate analysis. Adherence to the 2001 American Thoracic Society guidelines was high except in community-acquired pneumonia patients admitted to an intensive care unit. Length of stay was shorter in patients who received adherent rather than nonadherent antibiotic regimens.