Objective: The objective of our study was to investigate the prevalence of attention-deficit/hyperactivity disorder (ADHD) and comorbid psychiatric disorders in adults who presented to the psychiatric outpatient clinic, and the prevalence of ADHD-related clinical features described in the previous literature.Methods: The study was composed of two parts. We aimed to determine the prevalence in the first part. All patients who had presented to the Istanbul University Department of Psychiatry outpatient clinic during the period February-May 2005 had filled in the Adult ADD/ADHD DSM IV-Based Diagnostic Screening and Rating Scale. The patients scored above the cutoff value were evaluated with the Semi-structured Interview Form for ADHD and were diagnosed as having ADHD according to the DSM criteria by psychiatrist. The patients diagnosed with ADHD were reevaluated by consultant lecturer. In the second part of the study, comorbid diagnoses were investigated using the Structured Clinical Interview for DSM-IV Axis I Disorders (SCID-I) in the ADHD patients identified in the first part and the patients diagnosed with ADHD in the ADHD outpatient clinic, Department of Psychiatry, Istanbul Medical Faculty.Results: The patients diagnosed as having ADHD were mostly young men. Alcohol and substance abuse were very common problems inADHD patients. The prevalence of ADHD in adult psychiatric outpatient clinic was found to be at least 1.6 %. Conclusion: ADHD is represented less than the expected rate in the general population. ADHD symptoms should be inquired in adult patients who present to psychiatry departments with alcohol or drug problems. (Archives of Neuropsychiatry 2011; 48: 119-24)