Acute respiratory tract infection (ARI) in children is a prevalent condition that results in substantial morbidity and consumes large portions of health care resources in developing countries. We examined factors associated with the reported incidence and prevalence of ARI in a 3-year longitudinal study of 485 children < 5 years of age in rural Kenya. A large number of environmental factors, household and family characteristics, and child-specific factors were examined with use of multivariable methods. Few variables that may play a role in the incidence and prevalence of ARI (e.g., household tobacco use and weight z-score) were found to be related to such rates. Several factors related to ARI incidence and prevalence (e.g., mother's age, number of children, and community) were found to be associated with only mild ARI episodes. Evidence is presented to support the hypothesis that these factors are related to differential reporting of mild ARI episodes. The impact of such differential reporting on health care utilization and health education is discussed.