Cardiac patients who have social support generally have better prognosis than patients who lack social support. Several theoretical mechanisms have been proposed to explain this protective effect, including the capacity of social support to buffer the negative effects of stress. We tested this buffering effect in a study of patients hospitalized for acute coronary syndrome (ACS) in Spain. Several days after the cardiac event patients answered a questionnaire measuring stressful events during their lifetime, perceived social support around the time of the cardiac event, and depression symptoms in the past week. Results showed that stressful life events were related to depressive symptoms and worse renal function post-ACS only among patients with low perceived social support. Among patients who reported enough social support, lifetime stress was not related to depressive symptoms. No similar effects were observed on other prognostic indicators such as troponin levels or the number of obstructed arteries. These results suggest that social support can buffer the negative effects of stress on the mental and physical well-being of cardiac patients.