Extended-spectrum betalactamases (ESBLs) are recognized worldwide as a problem in hospitalized patients. Their prevalence among clinical isolates of Enterobacteriaceae varies between countries and institutions. We studied the evolution of ESBL production by clinical isolates of Escherichia coli and Klebsiella pneumoniae and analyzed the patterns of susceptibility of these isolates to different antimicrobial agents in a general university hospital in Beirut. Of the 4299 isolates of E. coli and 1248 isolates of K. pneumoniae tested over the five years, 2.0% of the E. coli and 20.0% of K. pneumoniae were ESBL producing. A clear decrease in the susceptibility to all antibiotics was observed between 1999 and 2001, and no resistance to imipenem was detected. The isolates were distributed between the Intensive Care Unit (ICU), medical wards, outpatients, and other origins. The highest numbers were found in the ICU (E. coli 28.1% and K. pneumoniae 34.8%). Three phenotypes of resistance to cefotaxime and ceftazidime were observed on the basis of microbiological results. The present study was the first to assess the occurrence and susceptibility patterns of extended-spectrum betalactamase-producing Enterobacteriaceae in Lebanon.