A fourth cohort of 330 preadolescent twin pairs stratified by sex and zygosity was recruited from an established population-based twin registry. The hemodynamic determinants of blood pressure were assessed by non-invasive measurements of cardiac function including echocardiography and response to isometric and dynamic exercise. Zygosity was determined by questionnaire and confirmed by dermatoglyphic analysis and blood group tests. Anthropometric and hemodynamic measurements in the parents were compared to those in the children. The study included up to five sets of longitudinal measurements, thereby permitting an evaluation of whether the same or different genes operated at different ages encompassing pre-puberty, puberty, and post-puberty.