The neurocognitive pathways by which placebo effects operate are poorly understood. Positron emission tomography (PET) imaging was used to assess the brain response of patients with chronic abdominal pain (irritable bowel syndrome; IBS) to induced intestinal discomfort both before and after a 3-week placebo regimen. A daily symptom diary was used to measure symptom improvement. Increases in right ventrolateral prefrontal cortex (RVLPFC) activity from pre- to post-placebo predicted self-reported symptom improvement, and this relationship was mediated by changes in dorsal anterior cingulate (dACC), typically associated with pain unpleasantness. These results are consistent with disruption theory [Lieberman, M.D., 2003. Reflective and reflexive judgment processes: a social cognitive neuroscience approach. In: Forgas, J.P., Williams, K.R., von Hippel, W. (Eds.), Social Judgments: Explicit and Implicit Processes. Cambridge Univ. Press, New York, pp. 44-67], which proposes that activation of prefrontal regions associated with thinking about negative affect can diminish dACC and amygdala reactivity to negative affect stimuli. This is the first study to identify a neural pathway from a region of the brain associated with placebos and affective thought to a region closely linked to the placebo-related outcome of diminished pain unpleasantness.