In an attempt to evaluate the histologic picture of what is shown on angiograms as neovascularity, several benign and malignant tumors were examined. New vessels (neovascularity) are large capillaries or sinusoids, and neither contain smooth muscle in their walls. The walls may contain some fibrous connective tissue. Puddling, laking, and staining represent the collection of contrast medium in small capillaries or sinusoids. Some tumors, such as hemangioma of the liver, cystadenoma of the pancreas, and angiomyolipoma of the kidney contained abnormal vascularity, but this was not composed of vessels which had arisen or developed anew, as there was smooth muscle in their walls, and this indicates that those vessels had been there from birth. In xanthogranulomatous pyelonephritis and chronic renal disease, the appearance of increased vascularity is the result of rearrangement of the normal vascular structures.