BK virus was first detected more than 30 years ago, however its role long time was not exactly defined. The virus is usually acquired in childhood; seroprevalence rate in adults appears to be 60-80%. In most cases the infection is asymptomatic. After renal transplantation it may manifest as so-called BK virus-associated nephropathy by gradual decrease of allograft function or signs of acute rejection. There is no effective therapy of BK virus-associated nephropathy to date. Treatment with low-dose cidofovir is under investigation.