Zidovudine (AZT) is a new anti retroviral agent widely used in the treatment of AIDS and related diseases. It has a short elimination half-life (1 h) in patients with normal hepatic and renal functions, and it is extensively metabolized to a glucuronide derivative (GAZT). In patients with end stage renal disease, the AZT elimination half-life is slightly increased (1.9 h) while for GAZT the very important increase of this parameter results in a major risk of accumulation. A 4-h hemodialysis is more efficient in removing GAZT than AZT. In these patients not only AZT but also GAZT plasma levels should be closely monitored.