The single breath N2-test and spirometry were carried out on two occasions 7 years apart in 460 men, 50 and 60 years old and sampled from the general population. We analyzed the relationship between the rate of decline in FEV1 during the follow-up and the steepness of the slope of phase III at the start of the study. The results showed an association between an increased rate of decline in FEV1 during the follow-up period and a steep slope of phase III at the start of the study, allowing for age, smoking habits and FEV1 at the start of the study (p less than 0.001). An increase in the slope of phase III from e.g. 1.0 to 4.0% N2/l in moderate smokers was found to predict an increase in the rate of decline in FEV1 by 50%.