Boys with hard lumps under the right nipple may be caused by breast hyperplasia, breast fibroids, breast cancer and so on. 1. Breast hyperplasia: due to the imbalance of endocrine metabolism in the body there is an increase in the level of estrogen, estrogen produces a continuous stimulation of the mammary glands, which can lead to proliferative changes in the breast tissue. 2. Breast fibroma: If the breast tissue under the right nipple is over-sensitive to estrogen, fibroma may appear. 3. Breast cancer: It may be manifested as a hard, irregularly shaped lump in the breast with unclear margins, mostly without pain. After diagnosis, it is necessary to receive comprehensive treatment mainly based on surgical treatment. It is recommended that boys with hard lumps under the right nipple should go to the hospital in time to clarify the cause of the disease and under the guidance of the doctor for standardized treatment.