(Preliminary methodical report). In situ hybridization of Chlamydia trachomatis in formalin fixed, paraffin embedded specimens from the Urology Clinic were performed in 2nd Department of Pathology Semmelweis University of Medicine by a biotin labelled DNA probe. Chlamydia trachomatis is suspected to be responsible for the chronic abacterial inflammation of the prostate besides Ureaplasma urealyticum and Mycoplasmae. According to our retrospective study out of 79 biopsy specimens 34 had the diagnosis of chronic abacterial prostatitis. We examined 11 specimens of them. Bacteria were not identified. Three specimens were positive for Chlamydia trachomatis which were improved by transmission electron microscope. The age of patients was between 59 and 81 years. The detection of chlamydia infection rises the suspicion of a "healthy" career and especially at younger age could cause Chlamydia trachomatis associated genital disease. The undiagnosed and untreated diseases may lead to infertility.