Hoarding Disorder (HD) is a common and chronic condition that is associated with distinct abnormalities in neural function. The impact of treatment on these abnormalities is not known. Six patients with HD and 6 healthy control (HC) participants underwent a functional magnetic resonance imaging (fMRI) task in which they made simulated decisions about whether to acquire or discard a variety of objects. The HD patients then received 16 sessions of manualized group cognitive behavioral therapy (CBT) for HD. After treatment, the HD patients completed the same fMRI decision-making tasks to identify changes in brain function elicited by decision-making. During acquiring decisions, reductions in HD patients' hemodynamic activity were observed in medial frontal gyrus, anterior and posterior cingulate, left insular cortex/postcentral gyrus, right caudate and putamen, paracentral lobule, bilateral posterior parietal lobe regions, right superior temporal gyrus, left hippocampus/parahippocampus, occipital cortex, and anterior cerebellum. During discarding decisions, reduced activity was observed in right superior/medial frontal gyri, left insula/putamen, posterior cingulate, precuneus, right posterior hippocampus, lingual/cuneus and right putamen. Prior to treatment, activity in these regions differed significantly between HD patients and HC participants. However, after CBT, most of these differences were no longer significant, suggesting normalization of neural function.