This study involved a detailed standardized initial research assessment which was carried out with 100 young people aged 12-15 years newly referred to a child and adolescent mental health service. The assessment involved the K-SADS interview with the young person and their parent, the Strengths and Difficulties Questionnaire, the Clinical Global Impression Scale, and the Children's Global Assessment Scale. Diagnoses resulting from these 'research assessments' were compared with clinical diagnoses, which were determined by case note analysis and discussion with the key clinician. Results showed that a clinical diagnosis of depressive disorder was made in only one-third of those who received a 'research assessment' diagnosis of depressive disorder, and suicidality was missed in a significant proportion of cases. Those with a diagnosis of depressive disorder had significantly more problems, more comorbidity, more suicidality and greater functional impairment than those without. It is important to keep depression and suicidality in mind when assessing young people with complex mental health difficulties. Unless specific pointers are sought, it is easy to miss these, which may mean that vulnerable young people do not benefit from potentially effective treatments.