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To examine possible deficiencies in endogenous pain modulating mechanisms in fibromyalgia patients compared with matched pain-free control subjects.Pain reduction was investigated in 25 female patients with fibromyalgia and 26 age-matched healthy women using the diffuse noxious inhibitory controls (DNIC) paradigm. Tonic thermal stimuli at painful and nonpainful intensities, tailored to individual heat pain thresholds, were employed to induce pain inhibition. The anticipated effect was assessed by measuring the electrical pain threshold and detection threshold, using a double staircase method. Only nontender control points were stimulated (thermode on the foot, electrodes on the inner forearm).The patients with fibromyalgia had significantly lower heat pain thresholds than the healthy subjects, but similar electrical detection and pain thresholds. The repeatedly applied electrical stimuli resulted in a degree of perceptual adaptation that was similar between the two groups. However, concurrent tonic thermal stimuli, at both painful and nonpainful levels, significantly increased the electrical pain threshold in the healthy subjects but not in the fibromyalgia patients. The electrical detection threshold was not affected in either group.Pain modulation, produced by a concurrent tonic stimulus in healthy persons, was not seen in the fibromyalgia group. The patients either had deficient pain modulation or were unable to tolerate a tonic stimulus intense enough to engage a modulatory process. It remains to be established whether the pain reduction found in the healthy subjects was the conventional DNIC effect, another effect (e.g., distraction), or a combination of both.