A population sample between 35 and 54 years of age with chronic obstructive lung disease was investigated. The material consisted of 86 persons with bronchial asthma (without chronic bronchitis), 69 with chronic bronchitis (without asthma) and 30 with a combination of chronic bronchitis and bronchial asthma. Of the 116 asthmatics 51 had reaginic bronchial allergy. Twenty-three subjects developed bronchoconstriction on exercise testing: 14 of these had asthma, one bronchitis and eight had a combination of the two. Onset of disease before 15 years of age was unusual in pure bronchitis (9%), although more common in pure asthma and in asthma with bronchitis (23-29%). The mean number of days of incapacity during the preceding 12 months was only 6, both in pure asthma and pure bronchitis, but it was more frequent, 22-26, in asthma with bronchitis and in exercise-induced bronchoconstriction. In asthma the multiple regression analyses showed that impaired spirometric values before or after taking a bronchodilating drug were correlated with the duration of incapacity in days and with exercise-induced bronchoconstriction. In bronchitis, impaired spirometric values were found to be correlated with the number of symptomatic days (during the preceding 12 months) and with smoking.