OBJECTIVE: The interplay between oxidative stress and inflammation is crucial in the pathogenesis of atherosclerosis. The adaptor protein p66Shc is implicated in atherogenesis and oxidative stress related responses in animal models of diseases. However, its role in humans remains to be defined. In this study, we hypothesized that expression of p66Shc increases in peripheral blood monocytes of patients affected by acute coronary syndromes (ACS). METHODS: Male subjects aged 59±4 (mean±SD) years admitted for cardiac catheterization were subdivided in three groups: (a) no local stenosis for the control group, (b) at least one stenosis ≥75% in either left, circumflex or right coronary artery for the coronary artery disease (CAD) group or (c) ST-elevation/non-ST-elevation myocardial infarction for the ACS group. Monocytes were isolated from whole blood and p66Shc RNA levels were determined by quantitative real time PCR. RESULTS: p66Shc RNA levels were increased in ACS patients as compared to CAD (p=0.007) and controls (p=0.0249). Furthermore, malondialdehyde (MDA) and C-reactive protein (CRP) were increased in plasma of ACS patients. Levels of MDA correlated positively to p66Shc (r=0.376, p=0.01). Our data demonstrate increased p66Shc levels in monocytes of ACS but not CAD patients. CONCLUSION: This study suggests an involvement of p66Shc in the transition of a stable CAD to an ACS patient. p66Shc was associated with states of increased oxidative stress. Further work is needed to understand whether p66Shc may represent a possible pharmacological target or whether it represents an interesting novel biomarker.