This study explored the comorbidity of DSM-III-R personality disorders in early-onset versus late-onset major depression in Japan. The subjects were 117 consecutive outpatients with major depression, with 26 classified as having an early onset (first depressive episode at age 22 or earlier) and 91 classified as having a late onset (first depressive episode at age 23 or later). Personality disorders were assessed using the Structured Clinical Interview for DSM-III-R Personality Disorders after a 2-month anti-depressant treatment. The results indicated that early-onset major depression was characterized by greater personality disorder comorbidity than late-onset major depression in Japan. Subjects with any one cluster A or B personality disorder were more prevalent in the early-onset group. In terms of each personality disorder, histrionic, narcissistic, and borderline patients were more prevalent, and the number of criteria met for schizotypal and cluster B personality disorders was significantly larger in early-onset major depression after corrections for age and gender. The results suggested that the higher prevalence of personality pathologies in early-onset major depression may reflect a higher likelihood to convert into bipolar disorders or a stronger impact of having experienced depressive episodes in young individuals. The possibility that the predisposing personality pathology may be different in early-onset and late-onset major depression is also discussed.