Introduction Competence is a prerequisite for informed consent. Patients who are found to be competent are entitled to accept or refuse the proposed treatment. In recent years, there has been an increased interest in studies examining competence for treatment in psychiatric patients. In this study, we aimed to investigate the decision-making competencies of inpatients with a range of psychiatric diseases. Methods This study was carried out at the psychiatry clinic of Kocaeli University Hospital in Turkey from June 2007 to February 2008. Decision-making competence was assessed in 83 patients using the MacArthur Competence Assessment Tool-Treatment (MacCAT-T). Results The study groups consisted of patients with mood (39.8%), psychotic (27.7%) and anxiety disorders (18.1%), and alcohol/substance addiction (14.5%). There was a significant relation between decision-making competence and demographic and clinical characteristics. Appreciation of the given information was more impaired in psychotic disorder patients than in other patients, but understanding and reasoning of the given information was similar in all groups. Discussion These results reveal the importance of evaluating decision-making competencies of psychiatric patients before any treatment or intervention is carried out to ascertain their ability to give informed consent to treatment. Institutional and national policies need to be determined and put into practice relating to the assessment and management of competence in patients with psychiatric disorders.