Infection by Chlamydia trachomatis was frequently observed in routine cytologic smears studied for cancer detection. Seventy-three smears from 187 Chlamydia-positive cases seen in a two-and-one-half-year period were reviewed to establish the relationship between C. trachomatis infection and the incidence of metaplastic and dysplastic cells. An inflammatory process with metaplastic cells was found in 72.6% of the smears and dysplastic cells in 16.4%. The Papanicolaou stain gave enough detail not only for the identification of the inclusions but also for the relation of different types of inclusions to the different stages of the life cycle of the microorganism.