A prospective study of Campylobacter jejuni infection was performed during a 6-month period in a cohort of 198 Chilean infants. Surveillance was based on biweekly home visits by a team of trained nurses. C. jejuni colonization was studied by culturing stool samples from diarrheal episodes and paired asymptomatic controls and by performing an enzyme-linked immunosorbent assay developed to measure C. jejuni outer membrane protein polyvalent antibodies in a representative group of infants. C. jejuni was isolated from 30 of 299 diarrheal episodes (10%) and from 17 of 304 samples from asymptomatic infants (6%). Significantly higher (P < 0.05) C. jejuni illness/infection ratios were found in infants less than 9 months old. Polyvalent antibodies to C. jejuni were found in 27 of 89 serum samples (30%). The lowest prevalence (4%) was observed among infants less than 9 months old, and the highest was found in patients older than 15 months (63%; P < 0.01). Data obtained by analysis of paired seru