ABSTRACT Enteropathogenic Escherichia coli (EPEC) induces a characteristic histopathology on enterocytes known as the attaching-and-effacing (A/E) lesion, which is triggered by proteins encoded by the locus of enterocyte effacement (LEE). EPEC is currently classified as typical EPEC (tEPEC) and atypical EPEC (aEPEC), based on the presence or absence of the EPEC adherence factor plasmid, respectively. Here we analyzed the LEE regions of three aEPEC strains displaying the localized adherence-like (LAL), aggregative adherence (AA), and diffuse adherence (DA) patterns on HEp-2 cells as well as one nonadherent (NA) strain. The adherence characteristics and the ability to induce A/E lesions were investigated with HeLa, Caco-2, T84, and HT29 cells. The adherence patterns and fluorescent actin staining (FAS) assay results were reproducible with all cell lines. The LEE region was structurally intact and functional in all strains regardless of their inability to cause A/E lesions. An EspF _U -expressing plasmid (pKC471) was introduced into all strains, demonstrating no influence of this protein on either the adherence patterns or the capacity to cause A/E of the adherent strains. However, the NA strain harboring pKC471 expressed the LAL pattern and was able to induce A/E lesions on HeLa cells. Our data indicate that FAS-negative aEPEC strains are potentially able to induce A/E in vivo , emphasizing the concern about this test for the determination of aEPEC virulence. Also, the presence of EspF _U was sufficient to provide an adherent phenotype for a nonadherent aEPEC strain via the direct or indirect activation of the LEE4 and LEE5 operons.