BACKGROUND: It is unclear whether pubertal status or timing of puberty explains the increase in depressive symptoms in girls during adolescence.MethodThis is a longitudinal study based on 2506 girls from the Avon Longitudinal Study of Parents and Children (ALSPAC). Self-reported depressive symptoms at 10.5, 13 and 14 years were assessed using the Short Mood and Feelings Questionnaire (SMFQ). Pubertal status (Tanner breast and pubic hair stage) and timing of menarche were derived from questionnaires administered from age 8 to 14 years. We used multivariable regression models to examine the relative contributions of pubertal status and timing in accounting for increases in level of depressive symptoms at 14 years. RESULTS: With increasing age, the association between breast development and depressive symptoms strengthened. Pubertal status (breast stage), rather than timing of menarche, was independently associated with depressive symptoms at 14 years. There was strong evidence for a linear relationship between breast stage and depressive symptoms at 14 years [increase in 0.17 s.d. (range 0.10-0.24) of depressive symptoms for advancement of each breast stage]. CONCLUSIONS: Depressive symptoms in mid-adolescence were more strongly influenced by breast stage than timing of menarche. This could imply that the female rise in depression during adolescence is due to increasing estrogen levels, and might explain why the gender difference in rates of depression emerges at this stage. Future research should be aimed at identifying the mechanism of action of pubertal change, including direct effects of pubertal hormones and indirect effects mediated by psychosocial factors.