Acyclovir is an efficient treatment in virus infections of the herpes family. Now that it has been in use for more than 10 years, neurotoxicity is a well-known complication, especially after the intravenous use . However, recently cases of neurotoxicity after the oral use in patients with terminal renal failure (TRF) on haemodialysis (HD) have been described [2–5]. This has been associated with high serum levels of the drug, representing an overdose and a significant modification of the dosage has been recommended in these patients [2, 4, 5]. HD has been shown to be an efficient method of elimination as well as a diagnostic method to differentiate between acyclovir neurotoxicity and encephalitic herpes .