There are only few studies of the association between preterm birth and risk of chronic lung disease in old age. The aim of this study was to assess the association between poor fetal growth, preterm birth, sex and risk of asthma and Chronic Obstructive Pulmonary Disease (COPD) in adulthood. We have followed up a cohort of all infants born preterm (<35 weeks) or with low birth weight (<2,000 and <2,100 g for girls and boys, respectively) and an equal number of controls in a source population of 250,000 individuals born from 1925 through 1949 in Sweden (6,425 subjects in total). Cases of asthma and COPD were identified through the Swedish Patient Register and we considered cohort subjects as cases if they had a main or additional discharge diagnosis of asthma or COPD. For any obstructive airways disease, there was a statistically significant increase in risk with decreasing birth weight and gestational duration among women but not among men. Compared to women born at term, women born before 32 weeks of gestation had a hazard ratio for any obstructive airways disease and asthma of 2.77 (95% CI 1.39-5.54) and 5.67 (1.73-18.6), respectively. Low birth weight and preterm birth are risk factors for obstructive airways disease also among the old, but the importance of these risk factors differs between the sexes.