analyses were performed to estimate the medial thickness of the pulmonary and renal arteries in 35 autopsy cases of complete transposition of the great arteries (TGA). Twenty-one autopsy cases of ventricular septal defect (VSD) and 33 autopsy cases of normal cardiovascular systems were also examined for comparative analysis.The medial thickness of renal arteries was not influenced by age and remained unchanged throughout life. However, there were significant differences in medial thickness among TGA, VSD and normal cases. Normal and VSD cases showed higher value of medial thickness than TGA cases. The results suggest retarded development of the arterial walls in TGA. A positive correlation was observed between medial thickness and blood pressure in both the pulmonary and renal arterial systems in TGA and VSD. One of the most important findings in the comparative analysis of TGA and VSD was that the media of TGA was more remarkably attenuated than that of VSD at the same blood pressure level in both the pulmonary and renal arterial systems. The results suggest an underdevelopment of arterial media in pulmonary and systemic circulation in TGA and a possible suppression of medial hypertrophy in TGA by some unknown factors.histometry; arterial wall; anomaly of great arteriesComplete transposition of the great arteries (TGA) has held the interest of many investigators as a congenital heart disease showing a unique morphology and many observations in the field of pathology have also been reported. However, such interest has been directed primarily toward problems of anatomical definition and classification, with very few studies concerned with an analysis of the relation between the cardiovascular system and hemodynamics in TGA.In this study, we have made histometrical measurements on the pulmonary and renal arterial systems in 35 autopsy cases and compared these results with both normal and VSD cases. Here, we report a unique characteristic of the thickness of the pulmonary arterial walls in cases of TGA which were found to correspond to the intra-arterial blood pressure.