In a longitudinal, epidemiologic study, we investigated the relation of a number of factors to the loss of ventilatory lung function over time and to the incidence of chronic obstructive pulmonary disease (COPD). Data on 759 men and 1,065 women 19 to 70 yr of age examined twice during a 13-yr period were analyzed. The decline rate in FEV1 was related to age and was greater in persons with lower FEV1 values and in taller persons. The clear relation to smoking was confirmed. Occupational exposure to dusts in men and to variable temperatures in women significantly increased the FEV1 decline rate. In men in blood group A, the loss of FEV1 was smaller than in men in the other blood groups. In women who had had 4 or more children, we observed a faster decline in FEV1. In the analysis of COPD incidence, we confirmed that age, baseline FEV1 value, and, in men, smoking habit are the most important predictors of disease. Among women, also, attacks of breathlessness were demonstrated to increase disease risk twofold (p less than 0.10), even after adjustment for other factors in the logistic regression model. We estimated the index of risk for COPD and predicted 63% of male and 67% of female cases in the top 20% of risk distribution.