This retrospective cohort study found that syringes prefilled with heparin flush solution caused an outbreak of Serratia marcescens bloodstream infection at an outpatient treatment center in Texas in 2007. The epidemiologic study supported this conclusion, despite the lack of microbiologic evidence of contamination from environmental and product testing. This report underscores the crucial contributions that epidemiologic studies can make to investigations of outbreaks that are possibly product related.