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The central serotonergic system has been implicated in the pathophysiology of panic disorder (PD) by evidence of abnormally elevated serotonin-turnover, reduced pre- and post-synaptic 5-HT1Axreceptor sensitivity and binding and clinical improvement during administration of agents that enhance serotonergic transmission. Polymorphisms in genes that putatively influence serotonergic neurotransmission increase the vulnerability for developing PD specifically in males. We tested the hypotheses that serotonin transporter (5-HTT) binding is elevated in PD subjects vs. healthy controls in regions where in vivo evidence exists for both elevated 5-HTT and 5-HT1A receptor levels in PD and investigated whether the extent of this difference depends upon gender. Volunteers were out-patients with current PD (n=24) and healthy controls (n=24). The non-displaceable com- ponent of 5-HTT binding-potential (BPND) was measured using positron emission tomography and the 5-HTT selective radioligand, ( 11 C)DASB. PD severity was assessed using the PD Severity Scale. The 5-HTT-BPND was increased in males with PD relative to male controls in the anterior cingulate cortex (F=8.96, pFDR=0.01) and midbrain (F=5.09, pFDR=0.03). In contrast, BPND did not differ between females with PD and female controls in any region examined. The finding that 5-HTT-binding is elevated in males but not in females with PD converges with other evidence suggesting that dysfunction within the central serotonergic system exists in PD, and also indicates that such abnormalities are influenced by gender. These findings conceivably may reflect a sexual dimorphism that underlies the greater efficacy of serotonin reuptake inhibitor treatment in females vs. males with PD. Received 4 April 2012 ; Reviewed 17 May 2012 ; Revised 19 June 2012 ; Accepted 21 June 2012