Two cases of severe neurologic symptoms after a Blalock-Taussig anastomosis are presented in detail. The histories of 116 patients with cyanotic congenital heart disease who had such an anastomotic operation at the Mayo Clinic were reviewed. There was a low incidence (less than 1%) of postoperative neurologic complications in patients without evidence of focal dysfunction of the central nervous system prior to operation. The review suggested an increased incidence of complications in those patients with a cerebrovascular abnormality before surgery. It is likely that these complications are related to alterations in the vertebrobasilar arterial system associated with this procedure.