Morphology of cavernous transformation of the portal vein is discussed on the basis of 5 case reports. On histological examination it can be shown, that in these cases a combination is present of chronic recanalized portal vein thrombosis and formation of paraportal hepatopetal collaterals. Formation of collaterals starts with new growth of primitive sinusoidal vessels originating from the portal vein and protruding into the paraportal tissue. Later on these vessels exhibit increasing venous differentiation up to the point of formation of varices very late in the development. The meaning of the veins of Sappey is discussed in regard to collateral circulation and to the formation of the new vessels mentioned.