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This study compared individuals with fibromyalgia (FM) and individuals with chronic low back pain (CLBP) on repetition-induced summation of activity-related pain (RISP). Fear of movement, pain catastrophizing and depression were examined as potential mediators of group differences. The sample consisted of 50 women with FM and 50 women with CLBP who were matched on age, pain severity and pain duration. Participants were asked to lift a series of 18 weighted canisters. In one trial, participants were asked to rate their pain after each lift. In a second trial, participants estimated the weight of each of the canisters. An index of repetition-induced summation of pain was derived as the change in pain ratings across repeated lifts. Analyses revealed that women with FM obtained higher scores on the index of RISP than women with CLBP. The heightened sensitivity to RISP in individuals with FM was not due to generalized hyperalgesia or a greater work output. Consistent with previous research, fear of movement was positively correlated with RISP. Pain disability was also associated with greater RISP, but not pain catastrophizing or depression. Discussion addresses the processes by which individuals with FM might have increased RISP responses. The findings of this study point to possible neurophysiological mechanisms that could help explain the high levels of pain-related disability seen in individuals with FM.Patients with fibromyalgia showed greater activity-related summation of pain than patients with chronic low back pain.