In a comparison between piglets (1-7 days old) with diarrhea and healthy piglets of the same age and 1 to 8 weeks old, 810 intestinal strains of Escherichia coli from 81 piglets from as many different herds in Sweden were investigated with regard to O-group, enterotoxicity, and possession of K88 antigen. A clear difference was found between the E. coli isolates from piglets with diarrhea and from representatives of healthy herds without diarrhea, with regard to (i) the homogeneity of strains in individual pigs and (ii) the distribution of O-groups, K88 antigen, and frequency of enterotoxicity. Strains from piglets with diarrhea showed a high frequency of O-group 149 (53%), enterotoxicity (61%), and K88 antigen (56%), while not more than 3% of the strains from healthy piglets of the same age harbored any of these characteristics. Of the isolates in O-groups 8, 64, and 115, 36% were enterotoxigenic. The corresponding data for O-group 149 and nontypable strains were 96 and 1%, respectively. Furthermore, K88 antigen was only found in O-groups 8 and 149. In O-group 149, 96% of the strains (n = 167) produced the K88 antigen as well as heat-labile enterotoxin. In contrast, strains producing heat-stable enterotoxin were mainly found in O-groups 8, 9, 64, 115, and 141. There was a significant difference in the frequencies of E. coli strains producing heat-labile enterotoxin between piglets with diarrhea and those without, whereas the pathogenic role of heat-stable enterotoxin-producing strains was less apparent.