Thyroid hormone levels were measured in 12 patients on continuous ambulatory peritoneal dialysis and in 18 patients on haemodialysis, all in stable clinical status. Values in patients on peritoneal dialysis were not different from those found in controls, whereas a highly significant decrease in T3 and r T3 was observed in patients on haemodialysis. These results are in contrast with the low T3 syndrome and the high r T3 levels of systemic diseases and malnutrition. The difference between the two groups of patients on dialysis suggests that thyroid abnormalities do not result from renal failure alone, nor from malnutrition, but from a disturbance in the calorie/carbohydrate ratio of the ingesta.