A case of renal vein and inferior cava thrombosis in a previously catheterized infant with cyanotic congenital heart disease is reported. The different origins of R.V.T. in newborns and infants are briefly discussed. The different factors that favour the beginning of thrombotic process (cyanotic heart disease diabetic mother, etc.) are also emphasized. Some considerations are made on the danger inherent to the use of intravascular catheters.