Muscle blood flow was studied in 28 patients after Blalock-Taussig anastomosis using the radioactive xenon clearance method. Insignificant differences in blood flow between the arms were found at rest, but there was a highly significant lowering of the peak blood flow on the operated side. Peak blood flow values in the first years after the anastomosis were close to the values in obliterative artery disease. The difference in maximum blood flow became less apparent beyond the first 5 years after the operation. A prolonged time of onset and duration of hyperemia after ischemic exercise were observed on the side of the operation. Statistically significant differences in circumference and length between the arms were measured and a lower strength of contraction of the affected hand was demonstrated. Abnormally low blood supply to muscles is thought to be responsible for the anatomical changes in the operated extremity.