The study explored whether there is differential efficacy for patients with panic disorder treated with sertraline initially (primary group) versus those switched (transfer group) after intolerance or nonresponse to imipramine in the context of the open 24-week treatment phase of a long-term maintenance/discontinuation study. Similar assessment and treatment procedures were used in the 2 groups and there were no concurrent cognitive behavioral interventions. A consistent pattern suggesting decreased efficacy for the transfer treatment group (n = 11, 68 +/- 25 mg/d) compared with the primary treatment group (n = 11, 70 +/- 25 mg/d) was found on response rates, univariate repeated measures analysis of variance and within group effect sizes in intent to treat, and completer samples. These preliminary findings concord well with clinical intuition but are contrary to findings in the treatment of depression. Replication studies seem warranted.