The frequencies of capsular serotypes among 703 Klebsiella strains isolated from the blood of hospitalized patients were determined. More than 90% of the isolates were typeable, with 69 of the 77 known serotypes being identified. Serotypes 2, 21, and 55, representing 8.9, 7.8, and 4.8% of all the isolates, respectively, were observed at a frequency significantly higher (P less than 0.05) than that for other capsular serotypes. Approximately 43% of the serotypes appeared at a frequency of less than 0.5%. Differences were found when the seroepidemiology of North American and European isolates was compared. The current findings indicate that a capsular polysaccharide-based vaccine against Klebsiella organisms is feasible and should be multivalent, eliciting antibodies directed against the 25 serotypes which make up approximately 70% of all the bacteremic isolates.