In 1974, a twin registry was established from members of the Kaiser Permanente Medical Care Program's Northern California region. In all, 9,821 twin pairs were enrolled. In 1978-1979, 434 pairs of female twins were recruited from the registry for a clinical examination of coronary heart disease risk factors. The clinical examination included an extensive health questionnaire, measurements of plasma lipids and serum chemistries, analyses of genetic markers for determination of zygosity, measurement of expired air carbon monoxide and serum thiocyanate levels, and a complete physical examination. During the ten years since the first examination, several new or more specific coronary heart disease risk factors have been recognized, including low density lipoprotein and insulin levels, and the anatomic distribution of body fat.