Breast reconstruction for breast deformity is significant not only for esthetic purposes but also from a psychological perspective. There have been a few reports on treatment of congenital simple absence of the mammary gland. For patients in puberty, even if they are in the middle of the growth phase, breast reconstruction is very important for the mental quality of life. In our two cases of congenital absence of unilateral mammary gland, breast reconstruction with a tissue expander worked well in terms of esthetic results and the psychological condition of the young patients. In our institute, operative indications are as follows: (1) a girl over 15 years old (this age is selected as breast growth can be determined at this time), (2) no endocrine-related disorders, (3) preoperative examination of breast MRI or US showing the absence or significant hypoplasia of mammary gland, and (4) a wish for breast reconstruction by the patient herself. For patients in the middle of the growth phase, silicone breast implant does not require a donor site and is easily adjustable in terms of volume to match the growth of the breast on the unaffected side by exchanging the silicone breast implant. Therefore, silicone breast implant is a better procedure than skin flaps with their accompanying large donor sites.