Background. In previous studies of subthreshold conditions, co-morbidity has been largely ignored. The purpose was to examine rates of co-morbidity among subthreshold disorders and between subthreshold and full-syndrome disorders for the major non-psychotic classes of disorders from DSM-IV.

Method. Participants came from the Oregon Adolescent Depression Project (mean age=16·6 years; females=52·1%). On the basis of a diagnostic interview (K-SADS), participants were assigned to eight subthreshold disorders (MDD, bipolar, eating, anxiety, alcohol use, substance use, conduct, ADHD).

Results. Of the 1704 adolescents in the analyses, 52·5% had at least one subthreshood disorder. Of those, 40·0% had also experienced a co-morbid subthreshold condition, and 29·9% of those had a second co-morbid subthreshold condition. Of those with a subthreshold, 36·4% also had a full syndrome. The subthreshold forms of externalizing disorders were co-morbid with each other. As expected, subthreshold anxiety was co-morbid with subthreshold MDD but subthreshold anxiety was also co-morbid with subthreshold alcohol, conduct, and ADHD. The pattern of co-morbidities was nearly identical for males and females.

Conclusions. The hypotheses that externalizing disorders would be co-morbid with other externalizing disorders and that internalizing disorders would be co-morbid with other internalizing disorders was partially supported. Co-morbidities between subthreshold disorders and between subthreshold disorders and full syndrome should impact future research and clinical practice. The assessment of subthreshold disorders needs to include the assessment of other subthreshold and full-syndrome conditions.