The effect of infection with Chlamydia pneumoniae on host messenger RNA expression in human monocytic cells with complement DNA microarrays was studied. The data chronicle a cascade of transcriptional events affecting 128 genes, many of which have not previously been reported to be affected by C. pneumoniae infection. Down-regulated genes are primarily associated with RNA and DNA metabolism, chromosomal stability, and cell-cycle regulation. Up-regulated messages include those for a variety of genes with important proinflammatory functions. Many of the up-regulated genes-including the hyaluron receptor CD44, vasoconstrictor endothelin-1, smooth muscle growth factor heparin-binding EGF-like growth factor, and fatty acid binding protein-4-had been previously described as linked to the development of atherosclerosis and other chronic inflammatory diseases. C. pneumoniae-infected monocytes can contribute to the development and progression of diseases for which acute or chronic inflammation has been shown to be important, such as atherosclerosis.