Preterm birth affects lungs in several ways but only few studies have follow-up until adulthood. We investigated the association of the entire spectrum of gestational ages with specialist care episodes for obstructive airway disease (asthma and chronic obstructive pulmonary disease, COPD) at age 18–50 years. We used nationwide register data on 706 717 people born 1987–1998 in Finland (4.8% preterm) and 1 669 528 born 1967–1999 in Norway (5.0% preterm). Care episodes of asthma and COPD were obtained from specialised healthcare registers, available in Finland 2005–2016 and in Norway 2008–2017. We used logistic regression to estimate odds ratios (OR) for having a care episode with either disease outcome. Odds of any obstructive airway disease in adulthood were 2–3-fold for those born <28 or 28–31 completed weeks, compared with those born full-term (39–41 completed weeks), persisting after adjustments. For individuals born at 32–33, 34–36 or 37–38 weeks, the odds were 1.1- to 1.5-fold. Associations were similar in the Finnish and the Norwegian data and among people aged 18–29 and 30–50 years. For COPD at age 30–50 years, the OR was 7.44 (95% CI 3.49–15.85) for those born <28 weeks, 3.18 (2.23–4.54) for those born 28–31 weeks, and 2.32 (1.72–3.12) for those born 32–33 weeks. Bronchopulmonary dysplasia in infancy increased the odds further for those born <28 and 32–31 weeks. Preterm birth is a risk factor for asthma and COPD in adulthood. The high odds of COPD calls for diagnostic vigilance when adults born very preterm present with respiratory symptoms.