The evaluation of pain is one of the major problems facing general practitioners and specialists in medicine. Although the source of pain can be usually be traced to specific abnormalities in a given organ system, some patients present with generalized pain syndromes, such as fibromyalgia, for which no specific source can be found. Some researchers have begun to consider that although there may be a somatic source of such pain at its initiation, over time the pain may be maintained or exacerbated by functional alterations in critical regions of the brain and spinal cord that are involved in pain processing or pain inhibition. This article describes the techniques currently used to measure regional cerebral blood flow (rCBF) in the brain by single photon emission computed tomography (SPECT) imaging, and reviews the SPECT and positron emission tomography literature concerning alterations in functional brain activity associated with pain in healthy individuals and in patients with chronic pain, including those with fibromyalgia. The article concludes by describing the implications of current knowledge about pain and abnormal functional brain activity in the understanding of the pathophysiology of fibromyalgia and in the development of therapeutic strategies to manage patients with this disorder.