Real-time ultrasonography was found to be useful in the diagnosis of hydronephrosis, renal calculi, and renal neoplasia in the dog and cat. One dog with hydronephorsis, one with renal calculi, and a cat with a renal neoplasm were scanned with a real-time ultrasonic scanner. Abnormal enlargement due to fluid accumulation was determined by ultrasonographic imaging in the case of hydronephrosis. The location and mobility of the renal calculus were observed during ultrasonography. An uneven contour and parenchymal masses in the neoplastic kidney were detected by ultrasonographic imaging of the cat. The findings were confirmed either by radiography or by postmortem examination. Previously, hydronephrosis could be diagnosed only by radiography, especially by excretory urography. Ultrasonography offers a noninvasive method for diagnosis of hydronephrosis. Localization of a urinary calculus at either the proximal end of the ureter or within the renal pelvis may be difficult by radiography. The ability of real-time ultrasonography to detect motion of the calculi within the pelvis offers an improved diagnostic capability to the veterinarian. The inability of radiography to determine the cause of renal enlargement is contrasted with the ability of ultrasonography to detect changes in tissue density that might be compatible with renal neoplasms