Nine patients with Moyamoya disease were followed by the authors between 1982 and 1991. The patients were investigated with transcranial Doppler sonography (TCD), angiography and clinically. Two patients were treated conservatively. Bilateral extra-intracranial (EC-IC) anastomosis was performed in 4 patients; in 2 patients bypass surgery was performed unilaterally. One patient with an aneurysm of the carotid artery presented a subarachnoidal haemorrhage. There was a good correlation between TCD and angiographical findings in all patients. In four patients indication for bypass surgery was established by TCD. All patients who underwent EC-IC bypass surgery improved clinically; the symptoms disappeared in three patients and improved markedly in two patients. Transcranial Doppler sonography is proven to be a reliable atraumatic tool in diagnosis, indication for treatment and postoperative follow-up in patients with moyamoya disease.