The mechanisms of spatial summation of pain (SSP) include pain coding dependent on impulse frequency and the number of recruited central neurons. However, SSP may also be influenced by pain inhibitory mechanisms, such as diffuse noxious inhibitory controls. Abnormal interactions between pain inhibitory mechanisms and SSP may be relevant for chronic pain conditions such as fibromyalgia (FM) and may help explain why widespread pain is characteristic for this chronic pain syndrome. The present study was designed to determine the difference of thermal SSP in the upper extremities between FM and normal control (NC) subjects, particularly within and across dermatomes of the hand. Fourteen NC and 19 FM subjects were enrolled in this study. SSP testing sessions involved immersion of each individual fingertip as well as stepwise immersion of the fingers, hands, and forearms in a hot water bath (46 degrees Celsius) for 5s and 20s. In addition, immersion of several fingertips across dermatome C(7)-C(8) was compared to progressive immersion of the index finger (dermatome C(7)). These experiments demonstrated significant spatial summation of heat-induced pain in both FM and NC subjects. SSP was most extensive within the fingers, and became negligible as the stimulus area increased above the hand. Furthermore, SSP was more pronounced within one dermatome such as that of the index finger than across several dermatomes of the hand. These results were similar for both FM and NC subjects. Thus, mechanisms of SSP, including possible inhibitory factors that limit this relevant pain mechanism, appear to be similar for both FM and NC subjects.