Association between prematurity/low birthweight and adolescent depressive disorder studied using a case-control design within a prospective cohort study of 2032 adolescents. Odds for depressive disorder were 11-fold (95% CI 2-62) higher for the premature/low-birthweight participants after regression adjustment for major confounding factors. For premature/low-birthweight females, cumulative rates of depressive disorder over 30 months were 15.2% (95% CI 11.1-20.5) v. 1.8% (95% CI 1.6-2.1) in those with normal deliveries. Physiological adaptations in utero before full term may be implicated causally in some cases of depression in adolescence.