Hypermucoviscosity, rmpA (regulator of mucoid phenotype), aerobactin (an iron siderophore), kfu (an iron uptake system), allS (associated with allantoin metabolism), and K1/K2 capsules are important virulence determinants in Klebsiella pneumoniae for liver abscesses. We determined the prevalence of these virulence factors of 50 nonrepeat K. pneumoniae isolates recovered from patients with primary liver abscesses who were treated at 2 medical centers in Taiwan. Virulence genes were surveyed by polymerase chain reaction analysis. The prevalence of hypermucoviscosity phenotype, plasmid-born rmpA, aerobactin, kfu, and allS genes revealed 96%, 100%, 100%, 100%, and 100% in 26 capsular K1 isolates; 90%, 100%, 100%, 0%, and 0% in 10 K2 isolates; and 79%, 86%, 93%, 50%, and 0% in 14 non-K1/K2 isolates; respectively. When injected into mice intraperitoneally, regardless of any capsule K serotype, K. pneumoniae isolates with hypermucoviscosity phenotype as well as presence of rmpA and aerobactin genes exhibited high virulence for mouse lethality (LD(50), <10(2) CFU). Without significant difference in the prevalence of expressing hypermucoviscosity phenotype and carriage of rmpA and aerobactin genes, these virulent non-K1/K2 isolates are as capable as K1/K2 isolates of causing primary liver abscesses.