Two hundred four appendiceal isolates for Escherichia coli from 146 patients with either inflamed appendices (IA) (110 patients) or noninflamed appendices (NA) (36 patients) were characterized. Strains with P fimbriae were detected in 27% of IA and 31% of NA whereas type 1C-fimbriated strains were found only in IA (13%). Four serotypes, three with K5 antigens (O18:K5, O25:K5:H1, and O75:K5:H-) and one with K1 antigen (O75:K1:H7), were isolated only from IA (20 [18%] of 110); O25:K5:H1 was the most common serotype (isolated from 11 IA [10%]). Fecal isolates from the patients with IA resembled their corresponding appendiceal isolates rather than fecal isolates from patients with NA; this finding suggests that colonization of the gut by virulent E. coli--such as a hemolysin-producing, type 1C-fimbriated, P-fimbriated O25:K5:H1 serotype--may be a prerequisite for the development of appendicitis.