In a longitudinal study of pulmonary function, we investigated the relationship between loss of forced expiration over time and a series of potential risk factors, as well as 4 other measures of pulmonary function at initial visit. Data on 1,912 adults tested twice, with an average of 4.7 yr between visits, were used. Of the potential risk factors examined, only age and smoking were consistently associated with increased loss of forced expiratory function in both men and women. Statistically significant differences among ABO blood types and among Pi types were also seen in women, although the direction of these differences was unexpected in that non-Z Pi variant phenotypes showed less decline in pulmonary function than did the most common Pi M phenotypes. Similar patterns of differences were seen in men, although these were not statistically significant. Of 4 initial-visit pulmonary function measures examined, only closing capacity at initial visit was consistently associated with subsequent loss of forced expiratory volume in one second (delta FEV1) in both men and women. The final predictive model for delta FEV1 accounted for a modest proportion of the total variation seen in this sample, however, limiting its potential use for predicting decline in pulmonary function over a 5-yr span in individual subjects.