The aim of this study was to describe a pseudo-outbreak due to Serratia marcescens associated with laboratory contamination, and also the epidemiologic and laboratory methods used to determine the source of contamination. An apparent increase in positive culture results for Serratia marcescens was observed in the Clinical Microbiology Laboratory of Kocaeli University Hospital between September and November 2007. An outbreak investigation including retrospective and prospective studies using chart review, environmental sampling and random arbitrary polymorphic DNA-polymerase chain reaction (RAPD-PCR) of randomly selected isolates were performed by the Infection Control Committee. Nine out of 67 strains belonged to a real infection. S. marcescens was also isolated from saline solution in the specimen processing area of the laboratory. It was recognized that the technician has been using the same stock saline solution for processing specimens without changing the injector. RAPD patterns of the clinical isolates were identical to the pattern of saline strain. The contaminated saline solution was discarded, the technician was trained and no additional cases of suspected contamination have been observed. This pseudo-outbreak emphasize the importance of the observation of specimen processing procedures and the training of laboratory workers.