Objectives. This thesis describes a study whose objectives were to establish the prevalence and co-morbidity rates of psychological problems in a population of adolescents with conduct disorder. In addition, it aimed to assess the number and type of cognitive errors made by these young people, and to examine the relationship between psychological disorders and cognitive errors. Design and Methods. All pupils in a residential school for children with conduct problems were invited to complete the Strengths and Difficulties screening Questionnaire. Those who completed the questionnaire were invited to attend an interview to complete the Development and Well-being Assessment structured diagnostic interview and the Children's Negative Cognitive Errors Questionnaire. The responses were analysed using correlations and a one-way ANOVA. Results. It was hypothesised that the presence of internalising psychological problems would correlate significantly with number of cognitive errors and that certain disorders would correlate significantly with specific cognitive errors. However, all of the hypotheses were rejected. Conclusion. Scores on the screening questionnaire and diagnoses from the structured interview do not correlate with the total number or type of cognitive errors reported by this sample of young people using the CNCEQ.