The incidence of cryptorchidism at 3 month-old boy is 0.8% which is close to that of an adult, and its diagnosis can be done at 3 months old. In case of cryptorchidism both MRI and USG are helpful to locate an undescended testis. If genitalia is pigmented too weakly, serum testosterone should be measured to rule out hypogonadism. Pathological changes of basement membrane and mitochondria in an undescended testis at 1 year-old boy suggests an earlier orchiopexy at least between 1 and 2 years at age, although if it is unilateral case an orchiopexy may be done by 4 years old. Fertility and malignancy are also discussed in cases with cryptorchidism.