The authors relate serological inquires (micro-immunofluorescent assay), for valuing the specifical antibody response anti-Chlamydia trachomatis, in acute urethritis. In 53% of cases a immunological response was present and in 18% of the cases was high and significant. Medium title was 1:28. In normal people the medium title was 1:10, and antibodies to Chlamydia were present in 8% of cases. The authors point out the importance and utility of a correct operative protocol for an efficacious diagnosis and for a correct therapy.