Based on seroepidemiological studies, Chlamydia pneumoniae has been suspected to play a role in atherosclerosis. The organism was searched for in atherosclerotic lesions by electron microscopy, immunohistochemistry, culture and PCR. It appears from published data and our own investigations that contrary to what could be expected, the sensitivity of PCR is much lower than that of the 3 other techniques. Moreover, PCR results are frequently conflicting. We therefore propose to interpret the results in the light of the biology of Chlamydiae characterized by alternate phases of activity and latency in inflammatory lesions followed by fibrosis and scarring, and ultimate degradation, the antigens being detectable for a considerable longer time than intact DNA.