Ultrasound diagnosis of horseshoe kidneys depends on the demonstration of an isthmus or band of renal tissue across the midline of abdomen connecting the lower poles of the kidneys. In a number of cases this band of renal tissue may evade ultrasonic detection. In a series of 11 patients with horseshoe kidneys, sonographic detection of renal isthmus was achieved in seven patients, but in all 11 cases there was demonstrable alteration of renal outlines. Instead of normal reniform shape, the kidneys appeared "inverted pyriform" or "inverted triangular". Detection of such an altered renal outline should raise suspicion of horseshoe kidney.