The aim of this study was to determine the prevalence of extended-spectrum beta-lactamases (ESBLs) in nosocomial bacteremia isolates of Escherichia coli, Klebsiella pneumoniae and Klebsiella oxytoca at Hacettepe University Adult Hospital in Ankara, Turkey. A total of 344 blood culture isolates of E. coli (n=244), K.pneumoniae (n=86) and K.oxytoca (n=34) were included in the study from January 2003 to November 2005. Only one isolate from one patient was tested in the study. The isolates with ceftazidime and/or cefotaxime MIC values >= 1 mu g/ml were tested by ceftazidime-ceftazidime/clavulanic acid and cefotaxime-cefotaxime/clavulanic acid Etest (AB Biodisk Solna, Sweden) strips and evaluated as ESBL positive if the ratio was >= 8. Of the isolates, 33% (74/224) of E.coli, 31.4% (27/86) of K.pneumoniae and 47% (16/34) of K.oxytoca were detected as ESBL producers by any kind of two strips. However, 5.4% (4/74) of Ecoli, 3.7% (1/27) of K.pneumoniae and 43.1% (7/16) of K.oxytoca ESBL-producing isolates could be detected only by cefotaxime-cefotaxime/ clavulanic acid strips. It is important to use cefotaxime-cefotaxime/clavulanic acid as well as ceftazidime-ceftazidime/clavulanic acid ratio for detection of ESBL types that preferentially hydrolyze cefotaxime. Since prevalence of ESBL production is high in nosocomial Ecoli and Klebsiella spp. isolates in our hospital, surveillance of antibiotic susceptibility patterns is important for the empirical treatment of bacteremic patients.