We propose to continue longitudinal studies of three populations in order to (1) determine the predictive value of the single breath N2 test in identifying the individual who is going to progress to chronic airflow obstruction; (2) determine the contribution of various host and environmental factors to the development of chronic airflow obstruction; and (3) obtain further information about how lungs age and what factors affect aging. The population groups are (1) smokers and nonsmokers who were first tested at an emphysema screening center in 1971/72 and subsequently tested in 1975 and 1977; (2) a random sample of a working population studied in 1974, 1976, and 1978; and (3) parents of 19 children with homozygous alpha1-antitrypsin deficiency studied annually from 1976 to 1978. Our data base will include (1) information, obtained by questionnarie, relating to respiratory symptoms, smoking habits, chest illnesses, occupation, residence and family history of chest problems; (2) pulmonary function tests; spirometric tests before and after a bronchodilator and the single breath N2 test - a test of small airway function; and (3) trypsin inhibitory capacity and phenotyping for alpha1-antitrypsin deficiency. Subjects will be tested either annually or biennially giving a total follow-up time varying between 8 and 12 years.