[en] One of the major radiation-induced effects identified in the CRHL Study has been the development of a serious life-threatening renal syndrome characterized by progressive glomerulosclerosis in perinatally irradiated dogs of the Segment II Study. This syndrome has been studied extensively in the laboratory in order to elucidate its pathogenesis and characteristic morphologic and functional parameters, thus providing the necessary information for appropriate evaluation of Segment III dogs. It has been shown that perinatal irradation causes an immediate interference with postnatal kidney development resulting in a permanent reduction in functional nephrons followed by a delayed, progressive glomerulosclerosis. Recent studies indicate that in at least some dogs receiving doses of 83 rads there is an arrest in nephron maturation and reduction in mature nephron numbers associated with a compensatory renal hypertrophy