Twenty-three transurethral resected prostate specimens with histologic evidence of chronic abacterial prostatitis were evaluated by colorimetric in situ hybridization for evidence of Chlamydia trachomatis. Intracellular chlamydia bodies were detected in seven of 23 cases (30.4%). In five of seven positive cases, chlamydia inclusion bodies were present within both epithelial cells and intraluminal histiocytes. In two of seven positive cases, extracellular chlamydia bodies were identified in conjunction with intraepithelial inclusions. In only one case were extraductal, extraacinar, intrahistiocytic inclusion bodies observed. Our study confirms previous reports implicating C trachomatis as an etiologic agent in chronic abacterial prostatitis, and underscores the applicability of DNA probes for the detection and identification of C trachomatis in routinely fixed, paraffin-embedded tissues.