Our objective was to examine epidemiological characteristics of hospitalized patients with imipenem-resistant Serratia marcescens. We performed a case-control study using data collected from computerized databases and chart review. Molecular typing by pulsed field gel electrophoresis of available isolates was performed. One hundred and ten patients had Serratia spp isolated during the 23-month study period. Twelve were infected or colonized with S. marcescens resistant or of intermediate susceptibility to imipenem. Eleven of the 12 patients were detected during a seven-month period between August 1994 and February 1995, suggesting the possible occurrence of an outbreak. However, the patients were admitted to different wards and services and, in eight patients, imipenem-resistant S. marcescens were isolated within 48 h or admission. None of the patients had epidemiological links within other institutions. The 12 cases were not more likely to have been exposed to beta-lactam antibiotics, including imipenem, than patients with imipenem-susceptible isolates. Six isolates were available for typing by PFGE; three were indistinguishable or closely related whereas each of the other three isolates were unique. In conclusion both the prevalence of imipenem-resistant S. marcescens and its unusual epidemiologic characteristics warrant further study.