Background: Klebsiella pneumoniae is a pathogenic bacterium causing nosocomial infections in particular severe respiratory tract infections. Little information is available on the antibiotic susceptibility of pulmonary isolates of Klebsiella spp. The aims of this study were to determine the antibiotic resistance patterns and prevalence of extended-spectrum β-lactamase (ESBLs) producing Kleb. Among the respiratory isolates and to detect the possible clonal outbreaks associated with them. Materials and Methods: The respiratory isolates of K. pneumoniae (n=33) received from two Tehran hospitals during 2002- 2005 were evaluated. Disk diffusion was used to determine the susceptibility of isolates to 14 antibiotics. Phenotypic confirmatory and double disk synergy methods were used to detect extended spectrum β-lactamase producing isolates (ESBLs). Respiratory isolates were then analyzed by multilocus enzyme electrophoresis (MLEE). Results: ESBL phenotype was detected in 75.75% of the isolates. The most effective antibiotic was imipenem followed by tazobactam/piperacillin. MLEE analysis revealed 13 electrophoretic types (ETs). The locus leucine-tyrosine peptidase showed the highest genetic diversity (0.733). Conclusion: These 33 respiratory isolates consisted of 16.5% Klebsiella pneumoniae. This rate is lower than the neighboring country, Turkey (35%). However, ESBL-producing strains belonging to different genetic lineages are serious concerns at Tehran hospitals. Carbapenem is still considered one of the most effective antibiotics against multi-drug resistant isolates. (Tanaffos 2007; 6(3): 20-25)