Ultrasonography has been used to assess renal bleeding after percutaneous renal biopsy in 57 infants and children. A perirenal hematoma was found in 10 patients i.e. an incidence of 16 per cent. An additional patient with heavy hematuria and temporary obstruction of the pelvis due to clot formation had an enlarged kidney and pelvicalyceal dilatation on ultrasound examination. We did not find a correlation between the clinical findings, the fall in hematocrit and the presence of a hematoma. Ultrasound can be regarded as the method of choice for monitoring renal bleeding in children who have undergone kidney biopsy.