BK computer virus were detected as the cause of hemorrhagic cystitis in these patients. bladder was performed. Then levofloxacin 1 x750 mg VGR1 intravenous and IVIG 0.5 gr/kg were started. But the hematuria did not decreased. In the first case, treatment with leflunomide was started, but patient died due to refractory AML and severe graft-BK computer virus was detected in 53 hematopoietic stem cell transplant recipients.6 ASCT were performed in three of our cases. Hemorrhagic cystitis developed after 3th month of transplantation in the first case and hemorrhagic cystitis developed after approximately 1 months after transplantation in the other 2 cases. BK computer virus was detected as the cause of hemorrhagic cystitis. The use of quantitative or real-time PCR to detect BK computer virus DNA has been shown to be useful in plasma or serum, less frequently in urine in following renal transplant recipients.7 In all of the three cases, BK computer virus was detected in urine by real-time PCR test. The reduction of immunosuppression was known to be effective in the treatment of BK computer virus infection. The reduction of immunosuppression was found to be successful in removal of viremia in a single-center study with 24 patients.8 Several agents with anti-BK Fas C- Terminal Tripeptide virus activity were demonstrated.9 The cidofovir is cytosine nucleotide analogue and it is an active agent against various DNA viruses including poliomyelitis viruses.10 In a retrospective nonrandomized study, 8 of 21 patients were treated with weekly low-dose cidofovir administration and immunosuppression reduction, only 13 patients were treated with reduced immunosuppressive therapy.11 In another study, clinical response was obtained in all adult patients treated with cidofovir (16/19, 84%); but only 9 of 19 patients (47%) were detected reduction greater than 1 log in the BK computer virus weight Fas C- Terminal Tripeptide in urine.12 In a study conducted by Cesaro (81%) of 27 patients had a complete clearance of BK computer virus associated hemorrhagic cystitis after the introduction of cidofovir on the average 37 days.13 Cidofovir treatment and the reduction of immunosuppressive treatment was started to the patient. 1 log decreased in BK computer virus weight after 1st month of cidofovir treatment. Hematuria was regressed in case 2. Leflunomide is usually a prodrug. Anti-metabolite of leflunomide is usually A77 1726. Both have immunosuppressive and antiviral activities.15 The mechanism of action against BK virus is unknown.16 In the case series, leflunomide improved in 23 of 26 patients with BK virus nephropathy.17 Leflunomide treatment was started in one of our cases; but the patient died because of refractory AML and severe GVHD after 4 days. IVIG contain antibodies against to BK and JC (John Cunningham) viruses. Three of the our patients received IVIG. Quinolone antibiotics have anti-BK computer virus activities.18 Levofloxacin was given as a treatment or as a prophylactic agent in the treatment of active BK Fas C- Terminal Tripeptide viremia in two randomized trials.19 BK virus was detected as the cause of hemorrhagic cystitis in our cases. The cidofovir treatment and the reduction of immunosuppressive treatment caused to improve symptoms and findings. They caused to decrease in the BK computer virus load. Intravenous immunoglobulin G contain antibodies against JC and BK computer virus. These viruses are ubiquitous in the general populace. But, these antibodies not be neutralizing.20,21 Other studies indicate that this anti-BK antibodies are not protective and these antibodies indicate an augmented humoral response to an inadequate cellular immune response.22 IVIG 0.5 g/kg IV was started in case 2 and case 3. But hematuria did not decrease in these.