Background: Differences in personality profiles were examined between children who differed in their co-morbidity of externalizing disorders: attention-deficit/hyperactivity disorder (ADHD) and conduct disorder (CD). Methods: 11- and 17-year-old male and female twins from a community sample were categorized as ADHD only, CD only, co-morbid CD-ADHD, and controls (no ADHD or CD) based on threshold and subthreshold CD and ADHD diagnoses assessed with structured interviews. Multivariate analyses were used to identify patterns of personality that differentiate these four diagnostic groups. It was hypothesized that significant differences would be found in the pattern of personality variables between participants in the co-morbid group, compared to the other three groups, and that these differences would hold across developmental periods. Results: As expected, the co-morbid group had a pattern of personality marked by higher Negative Emotionality and lower Constraint than the other diagnostic groups. This pattern was evidenced across gender and age cohort. Conclusions: An extreme personality profile may represent a liability toward the occurrence of ADHD and CD with more extreme profiles contributing to the occurrence of both disorders among boys and girls. Implications for treatment planning and theoretical development are discussed.