A post-biopsy arteriovenous (AV) fistula in a renal allograft was diagnosed using color-coded Doppler sonography. Because the patient was asymptomatic, no specific treatment was initiated. 15 months after the diagnosis was established, severe hematuria with obstruction of the bladder occurred. Angiography revealed a connection of the fistula to the renal pelvis. The patient was treated successfully by transarterial embolization of the fistula. As a conclusion, we suggest periodical follow-up examinations of asymptomatic AV fistulas in renal allografts using color-coded Doppler sonography during 3-6 months. When no spontaneous regression of the fistula can be observed, embolization therapy should be performed even when the fistula is asymptomatic, in order to prevent late onset of complications.