The present study used two single case studies to evaluate clinical improvement as a result of cognitive behavior therapy in panic disorder with or without agoraphobia (PDA). A special emphasis was placed on assessing the reduction in catastrophic cognitions, which are considered to be of prime importance in PDA. Multiple measures were employed, during pre and post treatment and at a six month follow-up. Data from self-report questionnaires, physiological measures and cognitive sampling during in vivo exposure were used. Convergent results demonstrated clinical improvement and supported the importance of catastrophic cognitions in the process of change for treatment outcome of PDA.