Ten Escherichia coli O157 strains isolated from cattle and children in Poland were investigated by the use of molecular biological methods. All strains possessed the intimin and enterohaemolysin genes and harboured the genetic determinants for Stx2 toxin (five isolates), Stx1 toxin (two strains) or both (three isolates). The genetic relatedness of the strains was examined by restriction fragment length polymorphism (RFLP) of chromosomal DNA digested with Xbal and Notl. Nine closely related RFLP patterns were observed. Comparison of bovine and human E coli O157 isolates based on the analysis of Xbal and Notl digested profiles showed that all strains belonged to one genetic cluster. These results indicate that cattle must be considered as a possible source of human E coli O157 infection in Poland.