Antidepressant drugs are commonly used to treat fibromyalgia, but there is little knowledge about their mechanisms of action. The aim of this study was to compare the cerebral and behavioral response to positive treatment effects of antidepressants or placebo. Ninety-two fibromyalgia patients participated in a 12-week, double-blind, placebo-controlled clinical trial with milnacipran, a serotonin-norepinephrine reuptake inhibitor. Before and after treatment, measures of cerebral pain processing were obtained using functional magnetic resonance imaging. Also, there were stimulus response assessments of pressure pain, measures of weekly pain, and fibromyalgia impact. Following treatment, milnacipran responders exhibited significantly higher activity in the posterior cingulum compared with placebo responders. The mere exposure to milnacipran did not explain our findings because milnacipran responders exhibited increased activity also in comparison to milnacipran nonresponders. Stimulus response assessments revealed specific antihyperalgesic effects in milnacipran responders, which was also correlated with reduced clinical pain and with increased activation of the posterior cingulum. A short history of pain predicted positive treatment response to milnacipran. We report segregated neural mechanisms for positive responses to treatment with milnacipran and placebo, reflected in the posterior cingulum. The increase of pain-evoked activation in the posterior cingulum may reflect a normalization of altered default mode network processing, an alteration implicated in fibromyalgia pathophysiology.