Occurrence of coronary heart disease (CHD) was assessed among the sibs of 309 men with fatal or non-fatal CHD, including 126 men who developed the disease before age 46 (young patients), and 183 men who developed it at age 46-55 (middle-aged patients), and among the sibs of 212 reference men. The risk of early onset CHD was about 3 times as great for the sibs of the young as for those of the middle-aged patients. The risk was greatest of all, or up to 10-fold over the expected value, for the sibs of those young patients whose father or mother had died of CHD before age 70, but the risk was distinctly increased also for the sibs of the other young patients. By contrast, the sibs of the middle-aged patients carried an appreciably increased risk of early onset CHD only when one of the parents had died from CHD before age 70. Information about the family pattern of CHD is helpful in identifying the individuals at high risk for premature CHD.