Experienced angiographers were asked to differentiate malignancy from inflammatory disease on arteriograms of 37 renal masses containing fine reticular neovascularity, as opposed to the coarser, more irregular vessels typical of hypernephroma. The rate of accuracy was only 58%. Distribution of vessels chiefly around the periphery of the mass was associated with a diagnosis of inflammatory disease in all 6 patients with this finding; this was the sole criterion which proved to be helpful in making the distinction. When arteriography reveals reticular neovascularity, differentiation between malignant and inflammatory renal masses may be difficult or impossible, and the term tumor vascularity should be avoided in such situations.