Background It is not yet known whether the risk of developing atopic dermatitis (AD) is influenced by preterm birth. Moreover, AD risk has not been assessed in a large sample of extremely preterm infants (<29weeks' gestation). Objectives To determine whether the risk of AD is influenced by preterm birth. Methods We investigated the relationship between gestational age (GA) and AD using data from two independent population-based cohorts, including a total of 2329 preterm infants, of whom 479 were born extremely preterm. Results There was a lower percentage of children with AD in the extremely preterm group compared with those born at a greater GA (Epipage cohort, 2-year outcome: 133% for 24-28weeks, 176% for 29-32weeks, 218% for 33-34weeks, P=002; LIFT cohort, 5-year outcome: 11% for 24-28weeks, 215% for 29-32weeks, 196% for 33-34weeks, P=011). After adjusting for confounding variables, a lower GA (<29weeks) was significantly associated with decreased risk of AD in the Epipage cohort [adjusted odds ratio (aOR) 057, 95% confidence interval (CI) 037-087; P=0009] and the LIFT cohort (aOR 041, 95% CI 018-090; P=003). Conclusions Very low GA (<29weeks) was associated with a lower risk of AD compared with higher GA (29-34weeks) and full-term birth.