The presence of plasma cells in endometrial tissue has been linked to Chlamydia trachomatis infection. The aim of our work was to determine the strength of the association between C trachomatis infection and plasma cell endometritis. C trachomatis infection was detected by polymerase chain reaction (PCR) or immunohistochemistry in 5 (24%) of 21 endometrial tissue samples with plasma cell endometritis and in 1 (4%) of 28 tissue samples with no evidence of plasma cell endometritis (P < .02). Patients with plasma cell endometritis were also more likely to have symptoms and signs consistent with upper genital tract chlamydial infection; thus there is an association between endometrial C trachomatis infection and the presence of plasma cells in the endometrium. The histopathologic finding of plasma cell endometritis should encourage further examination of the tissue sample for simultaneous chlamydial infection. Plasmid-based polymerase chain reaction and immunohistochemical staining of paraffin-embedded samples are useful methods for detecting C trachomatis in endometrial tissue.