The authors' epidemiologic studies of trachoma on Taiwan and experimental monkey eye infections with and without Chlamydia trachomatis immunization are reviewed for the data they provide on the pathogenesis of trachoma. These studies indicate that trachoma is an immunopathologic disease in which the more severe progressive trachoma infections with pannus and scar formation occur only after reinfection. This hypothesis is supported by a 10-year study of 32 family households that were followed with repeated clinical and laboratory observations. Although most cases of active trachoma healed spontaneously, there were 26 persons in nine families who developed clinical and laboratory evidence of 29 episodes of new trachoma eye infection. On the basis of the clinical disease, its persistence, and the laboratory findings, these 29 new infections could be divided into 10 primary, nine secondary, and 10 tertiary infections. Details of the different stages of infection leading to chronic trachoma are presented.