We report an unusual sonographic presentation of late-onset hemorrhagic cystitis caused by BK virus infection in an 8-year-old who had undergone hematopoietic stem cell transplantation for treatment of relapsed acute lymphoblastic leukemia. Sonography showed the presence of multiple bladder nodules in a background of diffuse bladder wall thickening. The dominant bladder nodule showed increase in size on serial sonograms and was vascularized on Doppler interrogation. Biopsy confirmed BK virus-associated cystitis. Recognition of this sonographic appearance in the appropriate clinical setting might obviate the need for biopsy in these children.