This report presents estimates of the prevalence of heart and other circulatory conditions of children and youths age 6 through 17 years in the noninstitutionalized population of the U.S. Estimates are based on diagnostic impression data from the direct, standardized examination findings of the Health Examination Surveys among national probability samples representative of the 23.8 children age 6 through 11 years in 1963-65 and of the 22.7 million youths age 12-17 years in 1966-70. Findings are analyzed by age, sex, race, geographic region, urban-rural residence, annual family income, and in relation to other findings. Among children, about 570,000 have a significantly abnormal heart or other circulatory condition. Among youths, more than 1 million have a significant cardiovascular condition. Among children nearly three-fourths of these conditions are considered congenital, while among youths nine percent are considered congenital. The prevalence of cardiovascular conditions is higher among Negro children and youths than among white groups, higher among those living in the South or in rural areas, and higher among the unschooled and poor. Survey data are presented in 15 tables. Statistical notes, definitions of demographic and socioeconomic terms, and medical history and physical examination forms are appended. (Author/RH)