We extracted lipopolysaccharides from 58 O-serogroup strains of Escherichia coli with phenol-water for use as antigens for an Ec-LPS array. The Ec-LPS array was made by dot-blotting of E. coli LPS on PVDF membrane. Commercial anti-E. coli O-serogroup antisera reacted with homologous O-serogroup LPS in Ec-LPS arrays. Convalescent sera of 6 patients with hemolytic uremic syndrome reacted strongly with O157 LPS when IgM and IgA antibodies in patient sera analyzed by Ec-LPS arrays. When IgG antibody was analyzed in this array, it was difficult to diagnose the O-serogroup because of the reactivity of patient sera against many O-serogroup LPS. These results match those by ELISA and western blotting. Compared to these serological techniques, Ec-LPS array appears superior to ELISA and western blotting in cost performance, time performance, and technical complexity.