To be eligible for the study, patients had to be aged 18 years or older and were required to have a weekly panic attack frequency of 1 or more per week during the month prior to intake into the clinic, and at least 1 full panic attack during the week prior to randomization. Each patient was given a physical examination and routine laboratory tests were performed. Urine toxicology results at intake were negative in each case for substances of abuse (mean ± SD time that urine was tested prior to randomization, 11 ± 15 days). The work-up also included urinalysis, electrocardiography, liver and thyroid function tests, blood cell count, and levels of serum electrolytes, glucose, and gonadotropin (for women). The current, principal diagnosis of panic disorder with or without agoraphobia was confirmed by administration of a semistructured diagnostic interview based on the DSM-IV (Structured Clinical Interview for DSM-IV, Anxiety Disorder Interview Schedule, or Mini International Neuropsychiatric Interview Plus).5- 7 Patients with mild current major depression were eligible for the study provided that the depression was not more clinically prominent than the panic disorder. Patients were required to not have taken psychotropic medications for at least 2 weeks prior to randomization. No patients had received long half-life benzodiazepines such as flurazepam, diazepam, or chlordiazepoxide prior to the study, and 1 had taken 0.5 mg of clonazepam as needed until 4 weeks prior to the study. One patient had been taking fluoxetine until approximately 12 months prior to intake. Subjects were excluded if they had a lifetime history of a psychotic disorder, a bipolar disorder, or a condition affecting central nervous system function (eg, epilepsy, severe head injury, meningitis). Women who were pregnant or lactating were excluded, as were women of childbearing potential who were not using an acceptable method of contraception (oral contraceptive, intrauterine device, progesterone implants or injections, barrier methods, and cream). In addition, patients were excluded if they had a substance abuse disorder within 6 months of the diagnostic interview, had previously failed an adequate trial of sertraline or clonazepam, or were allergic to either drug in the past. Patients with a suicide plan or suicidal behavior during the month prior to intake were not eligible and were referred for ongoing clinical care.