The first outbreak of infections caused by an SHV-5 producing strain of Klebsiella pneumoniae is reported. Within a period of 1 year and 9 months, multiresistant K. pneumoniae strains caused severe infections, mostly of the lower respiratory tract, in 22 patients. The strains were resistant to penicillins, third-generation cephalosporins, aztreonam, chloramphenicol, tetracycline and co-trimoxazole. The resistance determinants were transferable to Escherichia coli. All isolates produced a beta-lactamase with a pI of 8.2. Ceftazidime was hydrolyzed at this band. These characteristics, together with the resistance phenotype, are identical to those of a reference strain producing the beta-lactamase SHV-5. The K. pneumoniae strains of all patients were identical in their capsular serotype (K1), plasmid pattern and plasmid fingerprint after digestion with Dra I restriction endonuclease. We conclude that this outbreak was caused by the spread of one clone of K. pneumoniae producing SHV-5 beta-lactamase among patients of different wards. Our results indicate a real risk for failure of therapy by third-generation cephalosporins in intensive care patients due to SHV-5 producing pathogens.