In nine patients with chronic compartment syndrome, the intramuscular pressure and muscle blood flow during constant dynamic exercise was studied by the microcapillary infusion method and by the 133-xenon clearance technique. Although muscle blood flow was normal at the start of exercise, pain and impaired muscle function eventually developed; muscle blood flow decreased while muscle relaxation pressure increased. The changes of muscle blood flow could not be correlated with any change of mean muscle pressure during exercise. Eight months after fasciotomy the exercise test was repeated. Patients experienced no symptoms and the muscle relaxation pressure and blood flow during exercise were normal. It is suggested that chronic compartment syndrome is due to increased muscle relaxation pressure during exercise which causes decreased muscle blood flow, leading to ischaemic pain and impaired muscle function.