OBJECTIVE To describe the clinical, epidemiologic and microbiological features of a large outbreak of infection with a multiresistant Salmonella enterica serotype Typhimurium definitive type DT204b infection involving at least 392 people in five European countries. METHODS Icelandic public-health doctors responded to a report on an Internet news site of an outbreak of infection with a multiresistant strain of Typhimurium DT104 in England by contacting the Public Health Laboratory Service (PHLS) Communicable Disease Surveillance Centre (CDSC). An international alert was sent out through Enter-net. All strains from England & Wales, The Netherlands, Scotland and Germany, and 17 of the outbreak isolates from Iceland, were phage-typed, screened for antimicrobial resistance, and subjected to molecular typing. Hypothesis-generating interviews were conducted, followed by case-control studies performed in Iceland and England. RESULTS Isolates from cases in Iceland, England and Wales, The Netherlands, Scotland and Germany were identified as Typhimurium DT204b. The antimicrobial resistance pattern was ACGNeKSSuTTmNxCpL. All strains tested displayed an identical plasmid profile. Strains from five cases in England & Wales and five cases in Iceland possessed identical pulsed-field profiles. Although a common source was suspected, only Iceland implicated imported lettuce as a vehicle, with an analytic epidemiologic study (OR = 40.8; P = 0.005; 95% CI 2.7-3175). CONCLUSIONS The identification of international outbreaks, necessary for investigation and control, can be facilitated by standardized phage-typing techniques, the electronic transfer of molecular typing patterns, formal and informal links established through international surveillance networks, and the early reporting of national outbreaks to such networks.