Follow up of 38 boys with unilateral hypogonadism due to unilateral (35 boys) or bilateral (3 boys) cryptorchidism revealed that the normal testicle developed compensatory hypertrophy even before puberty. The hypertrophic testicle was in most instances larger than the normal range. The hypertrophy seems to be of functional importance as linear growth, skeletal maturation, sexual development, including penile size, and urinary 17-KS and gonadotropin excretion were normal in all the boys with unilateral hypogonadism or anorchia and contralateral testicular hypertrophy. (J Clin Endocr