A 13-year-old patient developed severe shock due to administration of a Yersinia enterocolitica-contaminated red blood cell concentrate. Y. enterocolitica (serotype O:9, biotype II) was cultivated from the residual blood in the blood bag and from a stool sample of the blood donor. In the donor's plasma immunoglobulin M (IgM), IgA, and IgG antibodies against Yersinia outer proteins (YopM, -H, -D, and -E) were found. Since the donor remembered a short-lasting, mild diarrhea 14 days prior to blood donation, a transient attack of Yersinia enteritis may be associated with a longer than expected period of asymptomatic bacteremia that causes contamination of donor blood. Serological screening for IgM antibodies against Yersinia outer proteins might offer a way to reduce the risk of transfusion-associated Y. enterocolitica sepsis.