Aim: The aim of the study was to assess the influence of decreasing gestational age on the risk of developmental delay in various domains at age 4 years among children born at a wide range of gestational ages. Method: In a community-based cohort, the parents of 1439 preterm-born children (24 0/7 to 35 6/7wks) and 544 term-born children (38 0/7 to 41 6/7wks') born in 2002 and 2003 completed the Ages and Stages Questionnaire (ASQ) when their child was 3 years 7 months to 4 years 1 month old. The prevalence rates of abnormal scores on the ASQ-total problems scale were compared in preterm and term-born children and the resulting odds ratios for gestational age groups were calculated and adjusted for social and biological covariates. Results: The prevalence rates of abnormal scores on the ASQ-total problems scale increased with decreasing gestational age: from 4.2% among term-born children to 37.5% among children born at 24-25 weeks' gestation (p less than 0.001). The risk of an abnormal ASQ-total score increased exponentially with decreasing gestational age compared with children born at term (odds ratio per week of gestation 1.14, 95% confidence interval 1.09-1.19). A similar exponential pattern was seen on all underlying ASQ domains, both before and after adjustment. Interpretation: The risk of developmental delay increases exponentially with decreasing gestational age below 35 weeks' gestation on all developmental domains of the ASQ. Adjustment for covariates did not alter the pattern of exponential increase in developmental risk with decreasing gestational age. We speculate that both direct perinatal cerebral injuries and tropic and maturational brain disturbances are involved. (Contains 2 tables and 1 figure.)