Escherichia coli strains isolated from sporadic cases of acute diarrhea in children and adults and from children without diarrhea were investigated for the presence of the pAA plasmid. Strains harboring the pAA plasmid were isolated at similar frequencies from children with (19.6%) and without (10.8%) diarrhea and from adults with diarrhea (11.8%). The genotypic and phenotypic virulence markers of these strains were further analyzed. Most of the strains were positive for EAST1 (73%), and this toxin was detected significantly more frequently in strains from children with diarrhea than in strains from adults with diarrhea (P < 0.05). Likewise, pic sequences were detected significantly more frequently in strains from children with diarrhea than in strains from adults with diarrhea (P < 0.005) and controls (P < 0.025). Furthermore, the association of pAA positivity (pAA ؉ ) and pic positivity (pic ؉ ) was more frequently found for strains from children with diarrhea than for strains from controls, indicating that pAA ؉ pic ؉ strains may represent a subset of pAA ؉ strains associated with disease in children. Most of the strains (82.5%) adhered to cells presenting the typical aggregative pattern. The frequency of occurrence of enteropathogenic E. coli (EPEC) serogroups in the strains from children with diarrhea was very high (56%), while none of the strains from adults with diarrhea belonged to EPEC serogroups. Extraintestinal virulence markers were very commonly found in strains from adults with diarrhea. The frequencies of occurrence of the adhesins AFA and SFA were significantly higher in strains from adults with diarrhea than in strains from children with diarrhea. More than one extraintestinal virulence marker was found in 58% of the strains from adults with diarrhea but in only 7.7% of the strains from children with diarrhea. Our results show that pAA ؉ strains isolated from children and adults with diarrhea present very different profiles when enteroaggregative E. coli virulence markers, serotypes, and extraintestinal virulence markers are considered.Several studies have reported on the association of enteroaggregative Escherichia coli (EAEC) with diarrheal disease in children (3,4,13,17,21,23,33). The EAEC pathotype has been defined by its aggregative pattern of adherence to tissue culture cells, but this definition seems to cover heterogeneous groups of strains, as indicated in volunteer studies (26) and investigations of the natural occurrence of the disease (3, 13, 14, 17). Nataro et al. (26) carried out a volunteer study with four different EAEC strains and showed that three of them failed to elicit diarrhea. Several epidemiological studies reported on an association of EAEC strains with sporadic diarrhea, either acute or persistent, and outbreaks in different geographic locations (3,13,17,21,23,33). Furthermore, Steiner et al. (44) reported on an association of EAEC with growth impairment even in the absence of diarrheal symptoms. However, in some case-control investigations, EAEC strains were isolated...