Impairment of glomerular filtration rate (GFR), renal plasma flow (RPF), and maximal urinary concentrating capacity was noted in the surgical and medical survivors of a controlled study of colon bypass. Statistically similar, but slightly higher mean values for GFR and RPF were observed in nonedematous, nonencephalopathic cirrhotic patients who had recently recovered from a bout of hepatic decompensation. Patient selection is considered to be the most significant factor in explaining these clinical observations which differ from earlier published reports. The experimental findings did not support the concept of renal injury causally related to chronic use of neomycin sulfate or protein restriction. Clinical improvement in hepatic encephalopathy following colon bypass was unrelated quantitatively or qualitatively to the intestinal microflora.