After developing the Ec-LPS array for Escherichia coli O-serogroup serodiagnosis (J. Jpn. Infect. Dis., 2007; 81: 26-32), we tested the array's usefulness in sera bled over 8 years ago from 24 patients with pediatric diarrhea. IgM and IgA antibodies in 20 sera among sera from the 24 reacted with a single LPS spot, making itpossible to diagnose the O-serogroup. IgG antibodies in almost all patient sera reacted with many LPS among the 58 O-serogroup LPS of E. coli. O-serogroup strains of E. coli isolated from the 24 patients numbered 15. Among 11 patients in who O-serogroups were serodiagnosed by both methods, 7 were diagnosed with the same O-serogroups. Based on these results, this array appears useful in the O-serogroup serodiagnosis of E. coil.