We sought to investigate prospectively the microbial etiology and prognostic indicators of 95 institutionalized elders with severe aspiration pneumonia, and to investigate its relation to oral hygiene in using quantitative bronchial sampling. Data collection included demographic information, Activity of Daily Living, Plaque Index, antimicrobial therapy, and outcome. Out of the 67 pathogens identified, Gram-negative enteric bacilli were the predominant organisms isolated (49%), followed by anaerobic bacteria (16%), and Staphylococcus aureus (12%). The most commonly encountered anaerobes were Prevotella and Fusobacterium species. Aerobic Gram-negative bacilli were recovered in conjunction with 55% of anaerobic isolates. The Plaque Index did not differ significantly between the aerobic (2.2 +/- 0.4) and the anaerobic group (2.3 +/- 0.3). Functional status was the only determinant of the presence of anaerobic bacteria. Although seven cases with anaerobic isolates received initially inadequate antimicrobial therapy, six had effective clinical response. The crude mortality was 33% for the aerobic and 36% for the anaerobic group (p = 0.9). Stepwise multivariate analysis identified hypoalbuminemia (p < 0.001) and the burden of comorbid diseases (p < 0.001) as independent risk factors of poor outcome. In view of the rising resistance to antimicrobial agents, the importance of adding anaerobic coverage for aspiration pneumonia in institutionalized elders needs to be reexamined.