We characterized two STEC O104 : H4 clinical isolates collected in Poland from a 7-year-old boy with haemolytic uraemic syndrome (HUS) and his nanny. This household outbreak began on 29 May 2011. Because of its time-frame, the outbreak was assumed to be part of the international STEC O104 : H4 outbreak that arose in Germany in May 2011. The two Polish isolates were Shiga-toxin-producing Escherichia coli (stx2 lpf) with enteroaggregative E. coli pathotype (aggR aap aggA), thereby sharing the unique virulence properties of the epidemic STEC O104 : H4 strain from the international outbreak. The Polish isolates were multi-drug resistant and carried bla(TEM), strA, strB, tetA, sul1 and sul2 markers together with the bla(CTX-M-15) gene for CTX-M-15 extended-spectrum β-lactamase. PFGE patterns and plasmid profiles of the Polish isolates and the epidemic STEC O104 : H4 strain corresponded closely. This finding suggested an epidemiological link between the Polish STEC O104 : H4 isolates and the international outbreak. Retrospective serological investigations proved person-to-person transmission of the epidemic STEC O104 : H4 strain from a father who had visited Dortmund, Germany, to his 7-year-old son in Giżycko, Poland. To the best of our knowledge, this is the first report of household transmission of Shiga-toxin-producing E. coli in Poland.