Background. Several sources of heterogeneity in major depression have been identified. These include age of onset, presence of co-morbid disorders, and history of childhood sexual abuse. This study examined these factors in the context of the contrast between onset of depression in young women before and after age 16. Method: Sampling was carried out in two phases. In the first, questionnaires were sent to women aged 25-36 in five primary care practices. Second-phase subjects for interview (n = 197) were drawn from three strata defined on the basis of childhood adversities. Interviews conducted and rated independently assessed (1) recalled childhood experiences, psychopathology and parental psychiatric disorder, and (2) adult personality functioning and adult lifetime psychopathology. Frequencies of predictor and response variables, effect estimates and their confidence intervals were weighted back to the general population questionnaire sample. Results: Compared with adult-onset depression, juvenile-onset adult depression was associated with co-morbid childhood psychopathology and peer problems, poor parental care, and childhood sexual abuse involving actual or attempted intercourse; in adult life there were higher levels of co-morbid psychiatric disorders, and personality dysfunction. The adult-onset depression group was characterized by a history of contact childhood sexual abuse without actual or attempted intercourse, and to a lesser extent, poor parental care. Conclusions: The juvenile- versus adult-onset distinction appears to be important to heterogeneity in adult depression, implicating different individual and environmental factors during childhood, and different mechanisms in adult life. © 2004 Cambridge University Press.