We report disorganization of background activity in the EEG of 49 patients with chronic renal failure. An electroencephalographic and biochemical study was performed in 20 patients before and after dialysis sessions. We did the same study before and after peritoneal dialysis in 14 patients. We analysed the EEG compared to biochemical blood abnormalities before and after renal transplantation in 15 patients. We observed worsening of the EEG after hemodialysis and peritoneal dialysis and improvement after renal transplantation, suggesting that in our patients the abnormalities in EEG caused by chronic renal failure were reversible. From our results, we found it difficult to establish correlations between the EEG and the biochemical blood abnormalities. However, there is statistical correlation between hipernatremia, improvement of metabolic acidosis, increase of the ratio of zinc to copper and worsening of the background activity in EEG.