Ninety-one Papanicolaou-stained vaginopancervical smears were destained and subjected to in situ hybridization with Chlamydia trachomatis DNA probe. At cytologic examination (Pap test), 71 smears showed changes suggestive of chlamydial infection, while remaining 20 were negative. At the control by in situ hybridization, the results of Pap test were confirmed in 85 out of 91 cases, two false-positive and four false-negative cases being detected. The sensitivity and specificity of Pap test, compared with in situ hybridization, were 95% and 89%, respectively. Like some recent reports, the present study confirms the reliability of Pap test in the detection of Chlamydia trachomatis and its possible relevant role in reducing the diffusion of the infection, when properly applied to mass-screening program.