In industrialized countries enterotoxigenic Escherichia coli (ETEC) is mainly diagnosed as a cause of travellers' diarrhoea, but it is also known to cause foodborne outbreaks. We report an outbreak of acute gastroenteritis caused by ETEC serotypes O92:H- and O153:H2 as well as Salmonella Anatum, which affected around 200 Students and teachers after a high-school dinner in Greater Copenhagen, Denmark, November 2006. A retrospective cohort study showed that consumption of pasta salad with pesto was associated with an increased risk of illness (attack rate 59.4%; risk ratio 2.6, 95% confidence interval 1.2-5.7). Imported fresh basil used for preparation of the pesto was the most likely source of contamination. Although ETEC is associated with travellers' diarrhoea in Denmark, this outbreak suggests that a proportion of sporadic ETEC infections might be caused by contaminated imported foodstuffs. To improve food safety further, it is important to target this poorly regulated and researched area.