Bipolar Disorders (BD) are often comorbid with disruptive behaviour disorders (DBDs) (oppositional-defiant disorder or conduct disorder), with negative implications on treatment strategy and outcome. The aim of this study was to assess the efficacy of quetiapine monotherapy in adolescents with BD comorbid with conduct disorder (CD). A consecutive series of 40 adolescents (24 males and 16 females, age range 12-18 years, mean age 14.9 ± 2.0 years), diagnosed with a clinical interview (Kiddie Schedule for Affective Disorders and Schizophrenia for School-Aged Children-Present and Lifetime Version [K-SADS-PL]) according to American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders, 4th ed., Text Revision (DSM-IV-TR) criteria were included. All the patients were treated with quetiapine monotherapy (mean final dose 258 ± 124 mg/day, range 100-600 mg/day). At the end-point (3 months), 22 patients (55.0%) were responders (Clinical Global Impressions-Improvement [CGI-I] score of 1 or 2 and CGI-Severity [CGI-S] ≤ 3 and improvement of at least 30% Children's Global Assessment Scale [C-GAS] during 3 consecutive months). Both CGI-S and C-GAS significantly improved (p<0.0001). Nine out of the 16 patients with suicidality (56.3%) had a reduction in this severe symptom during the follow-up. Nonresponders were more frequently males, and more frequently had an attention-deficit/hyperactivity disorder (ADHD) comorbidity. Eight patients (20.0%) experienced moderate to severe sedation and eight (20.0%) experienced increased appetite and weight gain. In these severely impaired adolescents, quetiapine monotherapy was well tolerated and effective in>50% of the patients.