Throat swab cultures and indirect hemagglutination assay (IHA) for Pseudomonas pseudomallei were done in 1000 randomly selected children at a large hospital in northeast Thailand. During 18 months, 17 children with melioidosis were admitted (0.46% of pediatric admissions excluding neonates born in the hospital). Throat swab was positive for P. pseudomallei in 8 of these but in none of 1000 control children. IHA seroprevalence rose at a conversion rate of 24% per year, from 12% in those 1-6 months old to a plateau at approximately 80% after age 4 years. No control child < 4 had an IHA titer > 1:160. The median titer in children with melioidosis was 1:80 (range, negative [in3]-1:5120). Specificity of IHA declined with age, but high titers (> or = 1:640) remained diagnostically useful. Thus, throat carriage of P. pseudomallei indicates active melioidosis. There is no evidence for an asymptomatic carrier state in children. Environmental exposure to P. pseudomallei in endemic areas begins when the child becomes mobile.