The pathogenesis of reflex sympathetic dystrophy--variously known as Sudeck's atrophy, causalgia, algodystrophy, and peripheral trophoneurosis--is not yet understood, and diagnosing and treating patients is difficult. We have prospectively studied 829 patients, paying particular attention to early signs and symptoms. In its early phase, reflex sympathetic dystrophy is characterised by regional inflammation, which increases after muscular exercise. Pain was present in 93% of patients, and hypoaesthesia and hyperpathy were present in 69% and 75% respectively. With time, tissue atrophy may occur as well as involuntary movements, muscle spasms, or pseudoparalysis. Tremor was found in 49% and muscular incoordination in 54% of patients. Sympathetic signs such as hyperhidrosis are infrequent and therefore have no diagnostic value. We found no evidence consistent with the presence of three consecutive phases of the disease. Early symptoms are those of an inflammatory reaction and not of a disturbance of the sympathetic nervous system. These data support the concept of an exaggerated regional inflammatory response to injury or operation in reflex sympathetic dystrophy.