Infection caused by metallo beta-lactamase (MBL)-producing bacteria have been increasingly reported in Japan in recent years. We investigated the prevalence, clinical backgrounds and molecular epidemiology of MBL-producing Serratia marcescens strains isolated from blood cultures at our university hospital between 1991 and 2000. Forty-three S. marcescens strains were isolated in the period, and the incidence reached about 2% total bacteria detection in 1998 and 1999. There were 13 ceftazidime-resistant strains (31%), and five of them produced MBL. MICs of imipenem (IPM) were 4-16 mg/ml, of which one strain was susceptible to IPM. Of the five MBL-producers, four were obtained from a tertiary emergency ward, the underlying diseases being either serious trauma or cerebrovascular diseases. The other was isolated from a patient who underwent kidney transplantation. S. marcescens was no longer isolated from patients after administration of aztreonam (AZT), implying that AZT was clinically effective. When analyzed genetically using the AP-PCR technique, the patterns were identical among strains isolated in 1996, 1997 and 1998 and those isolated in 1999 and 2000 respectively, suggesting that the same strains may have inhabited the hospital wards for prolonged periods. Countermeasures against nosocomial infection was undertaken thereafter, resulting in reduction of isolation frequency. It cannot be excluded that the resistant bacteria spread as the amount of carbapenem consumption increased. Prudent use of antibiotics is mandatory to prevent further dissemination of such MBL producers.