Abstract Nicotine, the addictive component of cigarette smoke, has been shown to have immunomodulatory effects. This drug alters proinflammatory cytokine production by immune cells, including lymphocytes, monocytes, and macrophages. The present study focuses on the effects of nicotine on infection by Chlamydia pneumoniae (Cpn), a ubiquitous intracellular pathogen which causes acute and chronic inflammatory diseases such as pulmonary infections, and may be associated with arthritis and atherosclerosis. Previous studies in our laboratory showed that lymphocytes and macrophages are susceptible to Cpn infection. The present study aimed at investigating the effect of nicotine on TGF-β1, IL-10, IL-12, and TNF-α production in Cpn-infected human peripheral blood mononuclear cells (PBMCs). Cytokine levels in the supernatant were assessed by ELISA. The results showed that Cpn infection alters the expression levels of IL-10, IL-12, and TNF-α in a time-dependent fashion. Nicotine treatment of the Cpn-infected cells up-regulated IL-10, but not TNF-α and IL-12, and also resulted in significant down-regulation of TGF-β1 production which was marked in the Cpn-infected control cells. The combined action of nicotine and Cpn on cytokine production may have an impact in chronic inflammatory diseases.