A representative sample of about 1000 70-year-olds in 1971/72 was included in the longitudinal and cross-sectional population study of elderly people in Gothenburg, Sweden (H70). The objectives were to make a survey of the social and medical conditions of this population, to obtain basic data for planning the care of the elderly, to contribute to the knowledge of normal aging processes and of normal criteria within the age group, and to offer a thorough medical examination. The survivors have been examined longitudinally at ages 70, 75, 79, 81, 82, 83, 85, 88, and 90 years of age. As possible cohort effects during 5-year-periods are also an object of interest, a further two cohorts have been examined at age 70, 75, and 79, and 70, 72, and 76, respectively. Another 75-year-old cohort has also been included (the Nordic comparative study of 75-year-olds-NK75). Using mainly identical methods, but different additional examinations, three Nordic countries cooperated in a comparative study of functional aging within the framework of the gerontological and geriatric population studies during the period October 1990-May 1991. The other centers were the universities of Copenhagen and Jyväskylä. The study aimed at comparing functional ability and biological aging between elderly in the Nordic countries. On an average, 3000 to 4000 variables about aging and diseases in old age are being collected per subject.(ABSTRACT TRUNCATED AT 250 WORDS)