An open trial of a therapist-guided internet cognitive-behavioral therapy (ICBT) for panic disorder with and without agoraphobia (PD/A) was conducted. Ninety adults diagnosed with PD/A were treated using ICBT adapted from a face to face (FTF) protocol. Results were benchmarked against two FTF samples, one at the same research site using the same protocol and another from a large cognitive-behavioral therapy (CBT) study. In addition, effects were compared to mean aggregated estimates from four meta-analyses. Attrition rates and therapist time were also examined to facilitate cost-effectiveness analyses and inform policy makers. Both full intent-to-treat and completer samples were used when analyzing data. Overall, results suggest that within-group effects for ICBT (0.88 to 1.7) are similar to the effects found in the benchmarking samples and to effects across meta-analytic studies. Effects were larger for symptoms assessed by an independent evaluator compared to self-report measures. Treatment gains continued to increase 3 months after post treatment and were maintained at 6 month and 1 year follow-up. However, attrition rates in ICBT were twice as large (46%) compared to the FTF sample, possibly due to a more conservative definition of attrition used here compared to previous reports. Therapist time in ICBT was reduced by a factor of three (14 min/week) compared to FTF, suggesting that treatment effects can be maintained even when reducing therapist time. Taken together, these findings suggest good short and long-term efficacy and time efficiency along with greater attrition for ICBT, allowing for dissemination and enhancing accessibility to quality, evidence-based treatment in the community.