Selective serotonin reuptake inhibitors (SSRI) have been established as effective drugs in the treatment of depressive and anxiety disorders. However, there are also reports that they can induce depressive symptoms and suicidal thoughts in patients. Eighty of 230 patients who met the DSM-III-R criteria for panic disorder received, during the course of treatment, fluvoxamine (a selective serotonin reuptake inhibitor) at a dose level between 50-200 mg/day. The patients were clinically evaluated for a history of affective disorder and for the presence of affective symptoms before the treatment and for emergence of depressive symptoms during the treatment. Seven of the 80 patients (9%) developed symptoms of depression despite a good antianxiety response. Five of the 7 patients received fluvoxamine as second choice after tricyclic antidepressants (TCA). These patients had no history of affective disorder, and no symptoms of depression were present before the treatment with fluvoxamine. The depressive symptoms abated after the fluvoxamine was discontinued and TCA or clonazepam was prescribed. The depressive symptoms reappeared when fluoxetine was administered. None of these 7 patients developed depressive symptoms while treated with TCA or clonazepam. Among the 150 patients treated with TCA and benzodiazepines, not a single case of depression was seen in patients without a previous history of depression. These results suggest a vulnerability among some of panic disorder patients to noradrenergic-serotonergic imbalance caused by SSRI, which has to be taken into clinical consideration.