L-carnitine is an important compound involved both in the transfer of activated long-chain fatty acids across the mitochondrial membrane and in the modulation of the acyl coenzyme A/free coenzyme A ratio in various intracellular compartments. These processes activate several metabolic and cellular functions, and L-carnitine supplementation proved able to reduce insulin resistance and improve lipid metabolism, muscle tropism and erythrocyte rheology. Thus, L-carnitine may be considered a conditional drug rather than a conditional vitamin. Several studies examined the effects of L-carnitine supplementation in dialysis patients. Most of these studies were performed in hemodialysis patients, with controversial conclusions: positive results were countered by the finding of minimal or even no effects. The results were, moreover, often biased by the small number of patients, the lack of control groups, the difference in the measured biochemical data, and the lack of evaluation of patients' compliance and intestinal drug absorption. In addition, at present it is still uncertain what is the adequate level of plasma carnitine to be considered as the target of carnitine supplementation in dialysis patients. In peritoneal dialysis, the small number of published papers provided controversial results as well. Some authors observed significant lowering of apolipoprotein B without changes in cholesterol, triglycerides, free fatty acids, phospholipids and apoliporotein A after administration for short periods of high-dose oral L-carnitine to adult or, more often, pediatric patients. Others did not observe any positive effect. In vitro studies demonstrated that peritoneal dialysis solutions containing Lcarnitine cause less damage to mesothelial and endothelial cells than glucosebased- only peritoneal dialysis solutions. More recently, L-carnitine was used as an osmotic agent in experimental dialysis solutions for human use: the peritoneal ultrafiltration was similar to that induced by glucose solutions. In addition, intraperitoneal administration allows the measurement of absorbed carnitine, thus establishing a correct dose/effect ratio. Ongoing clinical studies in peritoneal dialysis patients on the metabolic effect of glucose plus carnitine solutions, as compared to standard solutions, seem to suggest positive results.