There is serological evidence that a recently described new strain of Chlamydia, namely Chlamydia pneumoniae TWAR, is associated with coronary artery disease. This study describes the detection of TWAR organisms in the coronary arterial fatty streaks and atheromatous plaques in 7 autopsy cases. TWAR-like organisms were observed ultrastructurally in the lipid-rich core area of fibrolipid plaques and in intimal smooth-muscle cells. In 5 cases, immunoperoxidase staining showed positive reactions to Chlamydia genus- and C. pneumoniae species-specific monoclonal antibodies.