Chlamydia trachomatisinfection is the most common bacterial sexually transmitted disease. The authors examined the relation between chronic endometritis, which they term plasma cell endometritis (PCE), and chlamydial infection using plasmid-based polymerase chain reaction (PCR) and immunohistochemical staining (IHC) of paraffin-embedded endometrial sections.C. trachomatisinfection was detected in 5 (24%) of 21 cases of PCE and in 1 (4%) of histologically normal endometrium. The diagnosis of chronic endometritis (with plasma cells confirmed by methyl green pyronin staining) was correctly made in 74% of the cases originally diagnosed as PCE and in 23% of control cases originally diagnosed as normal. The authors conclude that the histopathologic finding of plasma cells in endometrial samples should encourage further examination for chlamydial infection.