Everyone encounters various stressors (causes of stress), such as psychological pressure, mental fluctuations, and physical burdens, in their everyday life. It is well accepted that the highest levels of perceived stress correlate with early onset of cardiovascular disease. Conversely, appropriate (mild to moderate) stressors, such as physical activity, have been shown to promote health. This bidirectional dose - response relationship of treatments that are beneficial at low levels but noxious at higher levels is referred to as "hormesis". In the fields of toxicology, pharmacology, radiation biology, and medicine, the significance of the biological effects of low-level exposure to various agents has attracted considerable attention. It is very important to understand how biological systems respond to low levels of stress and their implications within society. Aldehydes, the major endproducts of lipid peroxidation, have been implicated in the pathogenesis of oxidative stress-associated diseases. In addition to the pathogenic effect associated with oxidative stress, sublethal levels of aldehydes interact with signaling systems to upregulate the expression of genes to counteract the stressor challenge and to re-establish homeostasis. The present review article discusses current discoveries regarding the hormetic response to aldehyde and its clinical significance in cardioprotection.  (Circ J 2010; 74: 1787 - 1793)