Smoking cigarettes is the major risk factor for chronic obstructive pulmonary disease (COPD). COPD is a condition associated with chronic pulmonary inflammation, characterized by macrophage activation, neutrophil recruitment, and cell injury. Many substances contained in cigarette smoke, including reactive oxygen species (ROS), have been proposed to be responsible for the inflammatory process of COPD. However, this issue remains unsettled. By gas chromatography/mass spectrometry (GC/MS) we show that acrolein and crotonaldehyde, two α,β-unsaturated aldehydes, are contained in aqueous cigarette smoke extract (CSE) at micromolar concentrations and mimic CSE in evoking the release of the neutrophil chemoattractant IL-8 and of the pleiotropic inflammatory cytokine TNF-α from the human macrophagic cell line U937. In addition, acrolein (10–30 μM) released IL-8 also from cultured human alveolar macrophages and THP-1 macrophagic cells. 4-hydroxy-2-nonenal (30–100 μM), an endogenous α,β-unsaturated aldehyde that is abundant in lungs of patients with COPD, stimulated the release of IL-8 from U937 cells, whereas the saturated aldehyde, acetaldehyde, was ineffective. CSE-evoked IL-8 release was remarkably (> 80%) inhibited by N-acetyl-cysteine (0.1–3 mM) or glutathione monoethyl ester (1–3 mM). Both compounds, by forming covalent adducts (Michael adducts), completely removed unsaturated aldehydes from CSE. Our data demonstrate that α,β-unsaturated aldehydes are major mediators of cigarette smoke–induced macrophage activation, and suggest that they might contribute to pulmonary inflammation associated with cigarette smoke.