On January 30, 2009, nursing staff at a military hospital in Texas reported that single-patient use insulin pens were used on multiple patients. An investigation was initiated to determine if patient-to-patient bloodborne transmission occurred from the practice. Human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV) testingwas offered to patients hospitalized from August 2007 to January 2009 and prescribed insulin pen injections.Virus from HCV-infected patients' sera was sequenced and compared for relatedness. An anonymous survey was administered to nurses. Of 2,113 patients prescribed insulin pen injections, 1,501 (71%) underwent testing; 6 (0.4%) were HIV positive, 6 (0.4%) were hepatitis B surface antigen positive, and 56 (3.7%) had HCV antibody. No viral sequences from 10 of 28 patients with newly diagnosed and 12 of 28 patients with preexisting HCV infection were closely related. Of 54 nurses surveyed, 74% reported being trained on insulin pen use, but 24% believed nurses used insulin pens on more than one patient. We found no clear evidence of bloodborne pathogen transmission. Training of hospital staff on correct use of insulin pens should be prioritized and their practices evaluated. Insulin pens should be more clearly labeled for single-patient use.