1. Ultrasonic B-scanning is useful in differentiating between cystic and solid renal masses when the presence of a mass is known from plain roentgenograms or intravenous pyelograms, and the mass is known to be greater than 2 to 3 cm. in diameter. 2. Ultrasonic B-scanning is not a useful screening procedure. 3. Criteria for distinguishing a cyst from a solid lesion with ultrasonic scanning should be rigidly adhered to and all doubtful lesions should be presumed solid. An apparently normal renal echogram in the presence of a mass seen at intravenous pyelography suggests a solid lesion. 4. Comparison of the characteristics of the lesion with those of a reference organ avoids errors due to improper setting of the gain controls. 5. Ultrasonic B-scanning is useful in delineating hydronephrosis, when renal function is poor. 6. Ultrasonic B-scanning should follow the discovery of a renal mass lesion on intravenous pyelography and should precede either cyst puncture or renal angiography.