Ultrasonically-guided percutaneous nephrostomy using the Seldinger technique was attempted on 24 kidneys in 19 patiens with obstructive hydronephrosis. Seventeen of the procedures were successful, 4 failed and in 3 the catheter slipped out of the renal pelvis 1-48 h after insertion. The procedure is simple and independent of renal function. Postrenal uremia can be relieved, infected hydronephrosis drained prior to elective surgery and urinary parameters assessed. Injection of contrast material can define pathological anatomy in those cases where excretory urography and retrograde pyelography are unsatisfactory. No significant complications were encountered.