A sample of 300 pre-adolescent twin pairs was recruited from an established population-based twin study. The three cohorts of twins and their parents were initially evaluated in a cross-sectional study and then followed longitudinally for up to three years. In the initial testing cycle the following data were collected: genotype; general health; anthropometric measures; resting and exercise noninvasive evaluation of hemodynamic parameters including blood pressure, heart rate, cardiac output, left ventricular mass, and oxygen consumption; hematocrit and blood levels of 2,3-DPG, cotinine, thiocyanate, and erythropoietin; spirometric and pulmonary flow data; lipid levels. There were three follow-up exams. The availability of repeated measures of oxygen delivery and its determinants in twins and their parents permitted both a unique analysis of genetic and environmental factors during the process of developmental change and measurements of the risks of accelerated atherosclerotic/ischemic heart disease and of the development of reactive airway disease.