What causes itching in renal failure?

Increase urea levels. The kidneys are the cleansing organs through which all the blood flows in and dangerous by products of the body are filtered and removed through the urine. Everything the body needs is returned to the blood from the kidneys. As the kidneys fail, a protein breakdown product called urea builds up, and when high enough builds up in the skin causing itching. Transplant and/or dialysis may follow.
Itching in RF. The main causes in dialysis patients are: 1) drying of the skin due to accumulated waste products such as calcium, phosphorous, and urea not removed by dialysis; and 2) release of the body's internal substance called histamine due to the dialysis process. These are the typical reasons. Excellent source for more: http://kidney.Niddk.Nih.Gov/kudiseases/pubs/expect/#8.