BACKGROUND: Recent data from Korea showed that penicillin-resistance in pneumococci was more than 70% with 35% of multidrug-resistance (MDR) among invasive isolates. One of the most important reasons for the rapid increase of pneumococcal resistance in Korea would be the spread of resistance. Previous data of pulsed-field gel electrophoresis (PFGE) and penicillin-binding protein profile suggested the spread of pneumococcal resistance. To investigate the genetic relatedness of multidrug-resistant strains, we performed the ribotyping with resistant strains isolated from different countries. METHODS: A total of 42 pneumococcal isolates from Korea(33), Spain(5), and the United States(4), which were resistant to more than 3 classes of antimicrobial agents on agar dilution methods, and a R6 penicillin-susceptible strain were used for ribotyping. Ribotyping was performed with the restriction enzyme Pvu II by using a [alpha- 32P]dCTP-labeled gene probe from Escherichia coil 16S+23S RNA. RESULTS: Ribotype of a R6 strain was quite different form those of resistant strains. A total of 12 different ribotypes were noted in multidrug-resistant strains. Nineteen of 33 Korean strain (57.6%), 3 strains from the United States (75%), and 4 strains form Spain (80%) belonged to ribotype A or A subtypes. Discriminatory index of the ribotyping was 0.83. Ribotyping produced more patterns which could denote more discriminatory power than PFGE. CONCLUSION: The data strongly suggest the genetic relatedness of resistant strains from different countries. It might suggest the spread of pneumococcal resistance within Korea, which could partly explain the rapid increase of resistance in a short period.