During the spring of 2006, a national disease outbreak caused by Shiga toxin-producing Escherichia coli (STEC) O103:H25 was investigated in Norway. At the time of the outbreak the Norwegian School of Veterinary Science was the national reference laboratory for E. coli O157 in food, and the microbiological investigations to identify the food source were performed there. Food- and environmental samples (n=931) were collected by the Norwegian Food Safety Authorities following two different hypotheses i) that minced meat was the source of STEC, and ii) that fermented sausage was the source of STEC. Twenty seven food samples, all collected following the latter hypothesis contained eae-positive E. coli O103:H25, but none of these were stx-positive. By PFGE it was shown that isolates from one particular type of fermented sausage "morr sausage 1" were identical to the isolates from patients. Samples of sheep meat that were linked epidemiologically to meat used for sausage production also contained isolates identical or closely related to patient strains. The presented study underpins epidemiological indications that fermented sausage was the source of the outbreak, but points specifically to one particular brand of sausage as the source.