Objectives : Only a few prospective studies of panic disorder are available. This study investigated naturalistic outcome of panic disorder patients at twelve months after the initial diagnosis. Methods : A total of 84 subjects were diagnosed with panic disorder by diagnostic interview, Structured Clinical Interview for DSM-IV (SCID-IV) and Anxiety Disorder Interview Schedule for DSM-IV (ADISIV). Among them, 80 subjects could be evaluated by means of Panic Disorder Severity Scale (PDSS) at follow-up interview after twelve months. Treatment continuation was also examined at follow-up interview. Results : At initial intake, 80 patients were classified into 22% with mild, 33% with moderate-to-marked, and 45% with severe symptoms on the basis of their PDSS total score. At twelve months, 20% of patients reached remission, 65% had mild and 15% had moderate-to-marked symptoms. Initial panic symptom severity, presence of agoraphobia, panic symptom duration before diagnosis, number of comorbid Axis I disorders were associated with significantly higher PDSS total score at twelve months. Forty six percent of total patients continued medication and 23% have stopped treatment by clinician's recommendation and 31% have selfdiscontinued their medication. At twelve months, all three groups were improved but self-discontinuation group had significantly higher PDSS total score. Conclusion : In the one-year naturalistic outcome study of panic disorder patients, high percentage of patients achieved remission or had mild symptoms.