Thickening of the bladder wall and dilatation of the prostatic urethra can be shown by ultrasound in [humans] infants with posterior urethral valves. This can be helpful in the diagnosis of hydroureter. Marked hypertrophy of the bladder muscle can appear as a solid echogenic mass and simulate a pelvic tumor. Ultrasonic findings in 5 infants with posterior urethral valves are reviewed.