Three-phase bone scanning of the extremities (foot or hand) was performed in 40 normal subjects and in 56 patients with an unequivocal clinical diagnosis of reflex sympathetic dystrophy. Ten patients were in the "cold" or atrophic stage of the disease process, whereas 46 were in the "hot" or acute phase. The scintigraphic parameters studied were the ratios of tracer activity in the affected side over the healthy side established for blood flow (BF), blood pool (BP), early vasculo-tissular fixation (EF), and late bone fixation (LF). In the controls, blood flow, blood pool, and early fixation showed considerable interindividual variation and only the variation of late fixation remained within narrow limits. Among the patients, those at the hot stage of the disease had significantly higher values for all four parameters than those at the cold stage. The group at the cold stage did not differ from the controls except for a significantly higher late fixation value. Furthermore, among hot stage patients, 15% to 25% had normal or diminished blood flow, blood pool and early fixation values. At the cold stage of the disease, radionuclide parameters were similar in affected feet and hands, whereas at the hot stage values at the feet were double those at the hands. Finally, statistical analysis revealed that late fixation was most closely correlated with early fixation, which in turn was most close correlated with blood pool. The clinical and pathophysiological significance of these data is discussed.