Campylobacter species were isolated from 61 (15%) of 416 Thai children < 5 years old with diarrhea. Although the baseline levels of Campylobacter-specific antibody increased with age, 80.3% of Campylobacter-infected children seroconverted compared with 12.9% of 45 Shigella-infected patients used as controls. The response to acute infection was greatest in the 6- to 12-month-old group. Nonseroconverters had higher initial IgG levels than did seroconverters (P = .001). Quantitative cultures showed a range of 1-8 log10 Campylobacter cfu/g of stool (median, 6.0 log10), and the seroconversion rate was highest in those with the highest Campylobacter excretion. Fecal Campylobacter excretion was inversely related to age (chi 2 for trend, P = .03). These studies indicate that endemic Campylobacter exposure frequently induces seroconversion in young children, whether Campylobacter is isolated as a single pathogen or one of multiple pathogens, and that fecal excretion of the organism is inversely related to the age-related immune response to infection.