One hundred and fifty-four clinical isolates of Klebsiella pneumoniae resistant to broad-spectrum cephalosporins, aztreonam and amikacin were responsible for an outbreak of nosocomial infections lasting eight months in a university hospital in Paris. This outbreak occurred in the intensive care unit (39 patients), haematology units (8 patients) and surgical and medical units (11 patients). Antibiotic resistant strains were isolated from the urinary tract (48%), wound and drainage fluids (21%), respiratory tract (14%), blood (12%) and stools (5%). High resistance to oxyimino-beta-lactams was mediated by a plasmid-encoded beta-lactamase with an isoelectric point of 7.8 (SHV-4). This CAZ-type enzyme conferred a higher level of resistance to ceftazidime and aztreonam (geometric mean MIC 135 mg/l) than to cefotaxime (geometric mean MIC 14 mg/l). All isolates were of the same biotype (weakly urease positive and no sucrose fermentation). Eight Klebsiella pneumoniae strains isolated in different units and at different times of the outbreak were of the same serotype, had common plasmid patterns and harboured a large self-transferable plasmid of about 180 kilobases encoding resistance to penicillins, oxyimino-beta-lactams, aminoglycosides, tetracycline and trimethoprim. These eight large plasmids had indistinguishable EcoRI restriction patterns. These results suggest that a single strain of Klebsiella pneumoniae was responsible for this outbreak.