We studied the incidence of cervical Chlamydia trachomatis (CT) in 65 adolescents aged between 14 and 19 years and 65 adults aged 20 years or more. All subject were more than three months advanced in a normal pregnancy. Two samples for cytology were obtained from each patient, one ecto- and endocervical, with Ayre's spatula and cytobrush, and one only endocervical with cytobrush. The first one was examined after Papanicolaou staining and the second one was examined with the easily performed immunofluorescence reaction to CT (IF)--Microtrak, SYVA. Twenty seven adolescent patients (41.5%) and fourteen adults (21.5%) had a positive IF test for CT; the rate in adolescents was significantly higher (P < 0.01) than in adults. The Papanicolaou (Pap) stained slides, examined blind for evidence of CT infection showed a sensitivity of 70.7%, a specificity of 95.5% and positive and negative predictive values of 87.8% and 87.6% respectively. Seven adolescents showed cytological signs of Papillomavirus (HPV) infection, and six of them were also CT positive; five had signs of HPV infection and 4 of them were CT positive.