BACKGROUND: Klebsiella pneumoniae, a Gram-negative bacillus usually forming glistening mucoid colonies with viscid consistency on the culture plate, is a common pathogen causing various clinical infection patterns. However, little is known about the clinical implications of this mucoid character. OBJECTIVE: The purposes of this study, therefore, were to investigate the frequency of hypermucoviscosity (HV) in bacteraemic isolates of K. pneumoniae, and determine the significance of any association between HV and various clinical manifestations. DESIGN: Retrospective observational study. PATIENTS: Patients diagnosed with K. pneumoniae bacteraemia at a community-based university hospital between June 1999 and June 2001 were enrolled in this analysis. MEASUREMENTS: Clinical data concerning comorbid diseases and infection patterns was collected. K. pneumoniae bacteraemic isolates were examined for the presence of HV using a modified string test. The clinical impact of HV and risk factors for the invasive syndrome were assessed using statistical analysis. Polymerase chain reaction (PCR) was performed to detect magA, a gene related to HV phenotype. RESULTS: Overall, 200 (64.9%) of the 308 cases of K. pneumoniae bacteraemia were community-acquired infections. Compared with hospital-acquired K. pneumoniae bacteraemia (HA-KpB), community-acquired K. pneumoniae bacteraemia (CA-KpB) was more likely to be monomicrobial in nature (83.5% vs. 64.8%; P < 0.001) and caused by HV strains (41.5% vs. 14.8%; P < 0.001). The prevalence rate of magA among HV phenotypical K. pneumoniae strains was 24.1%. Patients infected with HV-positive strains were more likely to have the distinctive invasive syndrome (i.e. liver abscess, meningitis, pleural empyaema or endophthalmitis) than those infected with HV-negative variants (37.3% vs. 6.8%; P < 0.001). Multivariate logistic regression analysis, adjusted for age, showed that HV phenotype in K. pneumoniae strains (OR 8.86; 95% CI, 3.70-21.25; P < 0.001) was positively associated with the development of the invasive syndrome in CA-KpB cases. CONCLUSIONS: The HV phenotype of K. pneumoniae bacteraemic isolates was associated with the development of a distinctive invasive syndrome. Identification of the HV phenotype should prompt clinicians to initiate aggressive investigations for invasive diseases.