Reactivation of polyoma virus (BK virus) is a significant cause of morbidity in kidney transplant patients. This seemingly insignificant viral infection that affects the majority of population at a young age, once reactivated by immunosuppression, is a major factor contributing to graft loss. Screening techniques have been developed for early prediction of BK virus reactivation. These include plasma and urine assays for detection of BK virus DNA by PCR, urine cytology for detection of "decoy cells" and electron microscopy. Combining urine cytology and serology screening can be more effective for early detection of BK virus reactivation. Immunohistochemistry can be utilized as an additional tool to support the diagnosis. Once screening tests reveal a suspicious BK virus reactivation, tissue biopsy should be performed to confirm the diagnosis, rule out acute cellular rejection and plan treatment approaches. Treatment normally includes decreasing immunosuppression and the use of antiviral drug therapy. Unfortunately, disease outcome is often unfavorable and can culminate with eventual graft loss. Renal retransplantation has been performed with mixed results. As new data emerges, we will gain a better understanding of the disease caused by BK virus and respond with improved early diagnosis and treatment to preserve graft function.