Leptospirosis is an important public health problem in the Andaman Islands. The disease is being increasingly reported among children and adolescents in recent times. An attempt was made to find out the level of exposure to leptospires, to estimate the incidence of infection and to identity the risk factors for acquiring infection among children. A sample of 1544 schoolchildren was selected. Presence of anti-leptospiral antibodies was tested using the microscopic agglutination test (MAT). Students were interviewed for behavioural factors. In total, 341 (221 seronegative and 120 seropositive) students were followed up clinically and serologically during a subsequent outbreak. An overall seropositivity rate of 23·6% (95% CI 21·54–25·81) was observed. Infection rate was 33·5% among seronegatives whereas re-infection rate was 16·7% among seropositives during the outbreak that occurred 1 month after the first sample collection. Morbidity and mortality were found to be higher among seronegative individuals than serpositives, More than 90% of leptospiral infections were found to be subclinical or unnoticed. The high level of exposure among the children results in high infection rates and because they have less previous exposure than adults, they do not have sufficient protection to resist clinical illness during outbreaks.