We review the current knowledge concerning this newly recognized pathogen, Pittsburgh pneumonia agent (PPA), and present a new, comprehensive perspective of PPA based on our extensive clinical experience: 1) PPA pneumonia is more common and affects a broader range of patients than previously appreciated. 2) In the general population, the disease is not distinguishable from acute pneumonia due to other causes. Because specialized diagnostic tests are required for detection, it is likely that many cases in other hospitals go unrecognized. 3) Diagnosis is important, as erythromycin appears to improve outcome, whereas beta-lactam antibiotics and aminoglycosides, frequently used as empiric therapy for nosocomial bacterial pneumonias, do not. 4) The presence of both PPA and L. pneumophila in the same environmental sites, and the discovery of seven cases of simultaneous infection by both organisms suggest that both organisms are likely to share a common reservoir within the hospital and a common mode of transmission. 5) PPA infection occurs in a more debilitated population than does Legionnaires' disease. This may represent differences in intensity of exposure to the two organisms or may reflect inherent differences in virulence.