We describe a serious outbreak of infection caused by a strain of Serratia marcescens in two Dublin hospitals which occurred over an 11 week period and affected a total of 15 patients. A contaminated bed-pan macerator in the Intensive Care Unit of one hospital was identified as the possible source of infection and spread of the organism probably occurred via hand transmission by hospital personnel and via patient transfer to a second hospital. All isolates of S. marcescens involved in the outbreak had the same antimicrobial susceptibility pattern, with reduced susceptibility to gentamicin, cefotaxime and ciprofloxacin. Epidemiological typing revealed that the strains of S. marcescens isolated in the outbreak were of an uncommon serotype, O21:K14, and using pulsed-field gel electrophoresis, XbaI DNA macrorestriction profiles clustered at 90% similarity. The DNA patterns of the outbreak strain were also highly similar to S. marcescens isolates of the same serotype recovered from a separate Dublin hospital during the same time period as the outbreak described here. In addition, the isolates clustered at 82% similarity with strains of the same serotype from a retrospective collection of S. marcescens isolates from various hospitals in the Dublin area, indicating that these may be genetic variants of the same strain. Although the outbreak was brought under control following implementation of infection control measures, a significant number of similar O:21 isolates of S. marcescens have since been identified in four Dublin hospitals. These results suggest the unique spread of a single strain of S. marcescens in Dublin hospitals.