In November 1997, all Enterobacteriaceae isolated at 15 hospitals in Brooklyn were collected. Extended-spectrum beta-lactamases (ESBLs) were present in 44% of 409 Klebsiella pneumoniae isolates. Six isolates had reduced susceptibility to carbapenems, including two that were not susceptible to any of the antibiotics tested. Pulsed field gel electrophoresis revealed a commonality of resistant isolates within and between hospitals. The occurrence of ESBLcontaining isolates was associated with cephalosporin usage (P = 0.055). ESBLs were present in 4.7% of Escherichia coli and 9.5% of Proteus mirabilis isolates. It is concluded that ESBL-producing Enterobacteriaceae are endemic in Brooklyn, are spread between hospitals, and may be associated with cephalosporin usage.