['This study of a large clinical sample of depressed patients examined whether childhood onset as compared with adult onset Major Depressive Disorder (MDD) would confer a greater risk for Axis I comorbidity and whether childhood onset MDD would also differ from adult onset MDD in the pattern of comorbid disorders.', 'We examined lifetime co-occurrence of Axis I disorders among 381 adult outpatients with MDD by Structured Clinical Interview for DSM-III-R-Patient Edition (SCID-P). Subjects were divided into childhood onset (n = 47), adolescent onset (n = 101) and adult onset (n = 233) MDD groups.', 'We found that the two early-onset groups exhibited significantly increased rates of Axis I comorbidity. The childhood onset group accounted for a disproportionately high percentage of depressed adults with two or more comorbid Axis I disorders. Social and simple phobias and alcohol abuse/dependence were significantly more prevalent among individuals with childhood onset MDD than among individuals with adult onset MDD. Alcohol abuse/dependence, but not anxiety disorders, was significantly more prevalent among adolescent onset than adult onset MDD groups. Panic, generalized anxiety, obsessive-compulsive and somatoform disorders were equally distributed across MDD onset groups. Comorbid disorders were much more likely to have followed onset of MDD among individuals with childhood compared with adult onset, except for social phobia which more frequently preceded the depression. The relative ordering among the comorbid conditions with respect to whether they followed or preceded MDD did not vary notably across the three age of onset groups.'] Do patterns of axis I comorbidity in early-onset versus late-onset major depressive disorder?