A survey of the respiratory symptoms and smoking habits of a population of 20-year-olds- followed since birth- was repeated when they reached the age of 25. The association of cough prevalence with current smoking habits and with lower respiratory tract illness in childhood found in the survey at age 20 was confirmed and in each instance appears to have increased in strength over the 5 years. At age 25, however, the prevalence of cough was associated at a statistically significant level with fathers' occupation; and this association with social class of origin could not be explained by persisting differences in social status based on the educational levels attained by early adult life. The association with exposure to air pollution in childhood, although more obvious than before, could be due to chance. The prevalence of cough increased between the ages of 20 and 25 among those who smoked throughout or who started to smoke during this period. It declined for those who never smoked and for those who were smoking at 20 but had given up by 25. The implications of these changing patterns of respiratory disease behaviour at a crucial stage between adolescence and adult life are briefly discussed.