Histometric analysis of pulmonary vascular disease was performed in 21 nonshunted patients and in 13 shunted patients with tetralogy of Fallot and in 29 normal controls. There was no significant difference in the medial thickness of the small pulmonary arteries between cases of tetralogy of Fallot and normal controls. However, the media in the shunted cases of tetralogy of Fallot gave the impression of being thicker than these in the nonshunted cases. Intimal fibrosis, regarded as organized thrombi, and thrombi of small pulmonary arteries were observed generally after 4 years of age in shunted and nonshunted cases of tetralogy of Fallot. Intimal proliferation of musculoelastosis which was formed longitudinal smooth muscle bundles and elastic fibers was characteristic in shunted patients, especially after the central palliation procedure, Waterston anastomosis and modified Blalock-Taussig (B-T) anastomosis using a Gore-Tex tube graft. However, musculoelastosis was not usually seen in the B-T anastomosis cases. Unexpected pulmonary hypertension due to shunt operation is thought to be the cause of musculoelastosis, because musculoelastosis was observed even in a patient with pulmonary hypertension 3 weeks following surgery. We, therefore, recommend original B-T anastomosis as the shunt procedure. When considering use of the other shunt operations which entail pressure load on the pulmonary vascular bed, attention must be given to the size of the anastomosis or artificial tube graft.