The purpose of this study is to evaluate the effectiveness of Org 25935 vs. placebo given in combination with cognitive-behavioral therapy (CBT) to reduce the symptoms of panic disorder. It is hypothesized that treatment with Org 25935 at a dose of 4 mg or 12 mg will differ significantly from placebo with respect to the Panic Disorder Severity Scale (PDSS) total score over 3 weeks of therapy.