Chlamydia pneumoniae has recently been associated with atherosclerotic lesions in coronary arteries. To investigate the biological basis for the dissemination and proliferation of this organism in such lesions, the in vitro growth of C. pneumoniae was studied in two macrophage cell lines, peripheral blood monocyte-derived macrophages, human bronchoalveolar lavage macrophages, several endothelial cell lines, and aortic smooth muscle cells. Five strains of C. pneumoniae were capable of three passages in human U937 macrophages and in murine RAW 246.7 macrophages. Titers were suppressed in both macrophage types with each passage, as compared with growth titers in HEp-2 cells. Both human bronchoalveolar lavage macrophages and peripheral blood monocyte-derived macrophages were able to inhibit C. pneumoniae after 96 h of growth. Eleven C. pneumoniae strains were capable of replicating in normal human aortic artery-derived endothelial cells, umbilical vein-derived endothelial cells, and pulmonary artery endothelial cells. Infection in human aortic artery smooth muscle cells was also established for 13 strains of C. pneumoniae. The in vitro ability of C. pneumoniae to maintain infections in macrophages, endothelial cells, and aortic smooth muscle cells may provide support for the hypothesis that C. pneumoniae can infect such cells and, when infection is followed by an immune response, may contribute to atheroma formation in vivo. More studies are needed to investigate the complex relationship between lytic infection and persistence and the potential for C. pneumoniae to influence the generation of atheromatous lesions.