Seventy-eight non-invasive prostate specimens collected from patients with chronic non-bacterial prostatitis were evaluated by in situ hybridization (IH) for evidence of Chlamydia trachomatis. Intracellular Chlamydia bodies were detected in 18 of 78 cases (20.6%). Homogeneous blue-black bodies in the cellular cytoplasm were accepted as in situ positive. Chlamydial antigen detected by enzyme immuno assay (EIA) was positive in 12 cases (13.7%), but only nine of them were positive by IH. Our study confirms previous reports implicating C. trachomatis as an aetiological agent in chronic non-bacterial prostatitis, and underscores the applicability of DNA probes for detection and identification of C. trachomatis in prostatic materials.