Down patients frequently present congenital heart disease (CHD). Among these the most common is complete atrioventricular canal (CAV). The Authors studied by cardiac catheterization 2 groups of subjects with CAV:21 Down-patients, aged 5-48 months (mean = 24.7) and 17 non-Down patients aged 6-50 months (mean = 23.1). Hemodynamic data were compared and in particular pulmonary vascular resistances (PVR) were analysed. Mean pulmonary resistance was 11.1 UM2 for Down children and 4.6 UM2 for non-Down children. Considering the subjects under 2 years of age, PVR greater than or equal to 10 UM2 was present in 5 of 14 (35.7%). Down children, while none in the non-Down group. The cause of this behaviour is unknown; some Authors, in pathological studies, found in Down-patients primitive structural anomalies of the vascular bed with decrease of total vascular surface area. This fact might explain higher values of PVR in Down-patients than in non-Down patients.