Leflunomide (Arava) is an immunosuppressive drug, approved for the treatment of rheumatoid arthritis, and has been used in more than 300,000 patients worldwide. Furthermore, leflunomide has been used safely in humans after clinical kidney and liver transplantation for more than 300 days. In addition to leflunomide's value in preventing rejection, it has been shown to exert inhibitory effects on different viruses. Recently published pilot studies suggest that leflunomide treatment of patients with BKVN significantly reduces the amount of BK virus in blood and prevents recurrence of kidney transplant injury. At Karolinska University Hospital, leflunomide has been used for treatment of BKVN and, in some of the patients, renal function has stabilized and BK virus load has decreased significantly.