['Exercise testing, even when combined with radionuclide perfusion imaging, does not accurately predict future clinical coronary heart disease (CHD) in low-risk asymptomatic populations. We hypothesized that these tests would perform better in a higher-risk population with a high prevalence of occult CHD. Siblings of persons with premature CHD represent such a group in whom it would be advantageous to identify affected individuals before the occurrence of clinically manifest CHD.', 'Exercise thallium scintigraphy was performed in 264 asymptomatic individuals less than 60 years of age who had a sibling with documented CHD before age 60. Despite an average age of only 46 years at the time of screening, 19 of 264 siblings developed clinical CHD (sudden death in 1, myocardial infarction in 10, coronary revascularization in 8) over a mean of 6.2 years (range, 1 to 9 years) of follow-up. Abnormal thallium scans were observed in 29% of men and 9% of women, while abnormal exercise ECGs occurred in 12% and 5% respectively. Of men >/= 45 years of age, 45% had an abnormal exercise ECG, thallium scan, or both. In contrast, only 3% of women < 45 years of age had an abnormal test result. Although abnormal exercise ECGs and thallium scans were both predictive of future clinical CHD, the thallium scan was associated with a higher relative risk. After adjustment for age, sex, and exercise ECG results, the relative risk of developing clinical CHD was 4.7 for an abnormal scan. Siblings with a concordant abnormal exercise ECG and thallium scan had a relative risk of 14.5. These siblings were all men > 45 years of age at the time of screening and had a strikingly high incidence of CHD (6 of 12, 50%).'] Does exercise thallium tomography predict future clinically manifest coronary heart disease in a high-risk asymptomatic population?