If fatigue in multiple sclerosis (MS) is related to an abnormal activation of the sensorimotor brain network, the activity of such a network should vary with varying fatigue. We studied 22 patients treated with interferon beta 1a (IFNβ-1a; Avonex, Biogen, Cambridge, MA) with no fatigue (10) and with reversible fatigue (12). fMRI examinations were performed: 1) the same day of IFNβ-1a injection (no fatigue; entry), 2) the day after IFNβ-1a injection (fatigue; time 1), and 3) 4 days after IFNβ-1a injection (no fatigue; time 2). Patients performed a simple motor task with the right, clinically unaffected hand. At time 1, compared with entry and time 2, MS patients with reversible fatigue showed an increased activation of the thalamus bilaterally. In MS patients without fatigue thalamus was more activated at entry than at time 1. In both groups at entry the primary SMC and the SMA were more activated than at times 1 and 2. At entry and time 1, when compared to patients with reversible fatigue, those without showed increased activations of the SII. Conversely, patients with reversible fatigue had increased activations of the thalamus and of several regions of the frontal lobes. An abnormal recruitment of the fronto-thalamic circuitry is associated with IFNβ-1a-induced fatigue in MS patients. Hum Brain Mapp, 2007. © 2006 Wiley-Liss, Inc.