BACKGROUND: Typhoid fever, caused by Salmonella enterica serotype Typhi, remams an important public health problem in Korea, and asymptomatic chronic carriers play a role in the endemicity. However, the molecular studies of S. typhi isolates are very limited. We characterized clinical isolates of S. typhi by molecular and phage typing tools for the extent of genetic diversity and relatedness among the isolates. METHODS: A total of 49 S. typhi isolates from sporadic cases of typhoid fever were collected in 3 university hospitals in Seoul during 1992 to 1998 and examined for in vitro susceptibility to 14 antimicrobials by disk diffusion method, ribotyping using PstI restriction enzyme, and pulsed-field gel electrophoresis (PFGE) using XbaI and Vi phage typing. The distribution of the epidemiological types and genomic DNA relatedness were analyzed. RESULTS: Forty-five out of 49 isolates were susceptible to all drugs tested. Thirty-two out of 47 were typable by phage typing and 56.3% possessed the phage type El or Ml. Forty-nine isolates divided into 6 different ribotypes (A to F) and 19 different PFGE types (AO through A17, BO) by ribotyping and PFGE analysis, respectively. Based on the 3 typing systems, 32 isolates divided into 17 different epidemiological types. The E1-A-A12 (phage type-ribotype-PFGE type) was most prevalent (18.8Fo) and isolated only in 1998, but distributed in various areas of isolation. The next prevalent M1-A-A1 (15.6%) was isolated from 1992 through 1998. The genetic relatedness based on PFGE analysis revealed that F (coefficient of similarity) values are 0.64 to 1.0 and 0.52 for A subtypes and BO type, respectively. CONCLUSIONS: We conclude that the circulating S. typhi strains in Seoul city show considerable genetic diversity, whereas most of them seems to be clonally related.