A naturalistic study with no predetermined duration of treatment was undertaken in order to examine the effectiveness of cognitive therapy in the treatment of panic disorder. Seventeen patients diagnosed as having panic disorder according to the Structured Clinical Interview for DSM-III Personality Disorders received a mean of 18 individual cognitive therapy sessions. Patients with personality disorder or depression required a longer duration of treatment to become symptom-free. As measured by a self-report weekly panic log, the mean number of panic attacks was reduced significantly to zero at the end of treatment. There was a concomitant reduction in self-report measures of depression and anxiety. Further, there was a significant reduction on a measure of cognitive dysfunction during panic attacks. Treatment results were maintained at 12-month follow-up.