The risk of developing overt coronary heart disease is examined in relation to occurrence of non-specific electrocardiographic S-T and T-wave abnormalities (NSA-ECG) in the Framingham Study. In the course of follow-up, 14% of the 5127 men and women had or developed NSA-ECG without clinically apparent intervening coronary heart disease. During 30 years of surveillance, 760 men and 578 women developed a first overt clinical manifestation of coronary heart disease. NSA-ECG appears to be a hallmark of a compromised coronary circulation which predicted the occurrence of every clinical manifestation of coronary heart disease independently of known risk factors including hypertension, its chief determinant. Coronary morbidity and mortality was increased twofold in each sex. The more common T-wave abnormality alone carried a significant increased risk, although the combination of S-T and T-wave seemed most hazardous. Persons who develop NSA-ECG without other explanation warrant vigorous preventive management against coronary heart disease.