Measurement of the length of the opposite kidney before and after removal of a multicystic dysplastic kidney shows that: 1. Compensatory renal hypertrophy does not develop in utero. 2. Compensatory hypertrophy develops promptly after birth. 3. Enlargement is sufficient to make total renal mass approximately normal by the age of 1½ years. 4. Compensatory hypertrophy probably develops whether or not the nonfunctioning kidney is removed.