Trachoma is a progressive, blinding eye disease endemic in many parts of the world. In 1907 Halberstaedter and Prowazek1described the typical inclusion bodies and considered their component elementary bodies to be the etiologic agent of the disease. Soon thereafter, these same investigators2and others3,4demonstrated inclusion bodies of identical microscopic appearance in cases of a self-limited nongonorrheal conjunctivitis of the newborn. The source of infection for the newborn was promptly discovered in the adult genital tract by the finding of inclusions in the female cervix and the male urethra. Initially, both the genital tract involvement and the newborn eye disease were called "genital trachoma," but subsequent observations indicated that adult eye infections with the genital tract agent (acquired in swimming pools or by experimental inoculation) invariably ran a short, self-limited course without scars or pannus formation. Thus, the concept arose that the agent of inclusion conjunctivitis,