Background: Patients with panic disorder (PD) show alterations of the immune reactivity to acute stress, which could serve as a marker for effective treatment. Nevertheless, the effect of immune reactivity under acute stress before treatment on therapy outcome remains unclear. Methods: A total of N = 16 PD patients performed the Trier Social Test. Blood sample collection of anti-inflammatory cytokine IL-10 accompanied the TSST. The Mobility Inventory was handed out for the assessment of avoidance behavior before and after treatment. Area under the curve with respect to the ground (AUC_G) and increase (AUC_I) were calculated for assessed cytokine levels and were used as predictors for therapy outcome in regression analyses. Results: AUC_G significantly predicts avoidance behavior in company after treatment (β = −0.007, p = .033) but not avoidance behavior alone (β = −0.003, p = .264). AUC_I does not significantly predict therapy outcome. Conclusion: Higher concentrations of anti-inflammatory cytokine IL-10 under acute stress before treatment predicts less avoidance behavior in company after therapy. Immune markers seem to play a crucial role in the maintenance of mental disorders such as PD. Underlying mechanisms and IL-10 as a marker for individualized treatments should be investigated in future studies.