An important subgroup of protein-bound toxins are generated as a result of protein fermentation in the colon. P-cresol is a fermentation metabolite of tyrosine. In renal failure, the colonic generation rate of p-cresol is markedly elevated. After absorption, the majority of p-cresol is conjugated to form p-cresyl sulphate. There is clear evidence, both in vitro and in vivo, that accumulation of conjugated fermentation metabolites is correlated with clinical important endpoints. Free p-cresol is an independent predictor for mortality in hemodialysis patients.