The evolution and dissemination of extended-spectrum $eta$-lactamases (ESBL) have compromised the clinical use of third-generation cephalosporins worldwide. Although most ESBLs belong to the TEM and SHV $eta$-lactamase families, the members of CTX-M, a novel ESBL family, are increasing worldwide in Gram-negative bacteria. We examined the prevalence of CTX-M ESBL in clinical isolates of the family Enterobacteriaceae collected from three university hospitals located in three different cities in Korea. Among a total of 603 isolates collected, 163 isolates (27.0%) revealed ${geq}2;{mu}g/ml$ of MIC against cefotaxime, and 93 isolates (15.4%) produced ESBL confirmed by the double disk synergy test. Among 93 ESBL-producing isolates, $bla_{CTX-M}$ genes were detected in 41 isolates by PCR method and they included 1 isolate of C. freundii, 3 of E. aerogenes, 2 of E. cloacae, 17 of E. coli, 9 of K. pneumoniae, and 9 of S. marcescens. Thus, the overall prevalence of CTX-M ESBL-producing isolates among the family Enterobacteriaceae was 6.8% (41 of 603 isolates) and the proportion of CTX-M-producers among the ESBL-producing isolates was 44.1 % (41 of 93 isolates). Further determination of the $bla_{CTX-M}$ subtype by nucleotide sequencing revealed $bla_{CTX-M-3}$ in 17, $bla_{CTX-M-15}$ in 11, $bla_{CTX-M-14}$ in 9, and $bla_{CTX-M-9}$ in 4 isolates. To our knowledge, this is the first report on the dissemination of CTX-M ESBL among clinical isolates of the family Enterobacteriaceae in Korea.