Pyelitis - an inflammation of a renal hollow. The acute inflammation of a mucosa of the top urinary ways - hollows and calyxes - is invariable accompanied by inflammatory damage and parenchyma of kidneys. The chronic inflammation of a hollow and calyxes is always supported by any reason - a chronic inflammation of an intersticial tissue of kidneys or the disease interfering liquidation of inflammatory process (a stone of a hollow, a tumour of the top urinary ways, a stricture of a ureter, etc.). In all these cases it is necessary to diagnose a basic disease - the chronic pyelonephritis, the infected nephrolithiasis, a hydronephrosis, a tumour, etc. the "Pure" pyelitis practically does not happen, and to speak about an acute and chronic pyelonephritis more correctly.