A hard lump of peanut size in the breast may be related to breast lipoma, breast fibroma, breast cancer and so on. 1. Breast lipoma: it may be related to bad dietary habits and belongs to benign tumors, usually grows under the skin of the breast, and the location of the lump is shallow, the boundary is clearer, and it is soft to the touch. 2. Breast fibroma: it is a benign tumor, usually related to endocrine dysfunction and other conditions, the typical symptom is the appearance of small hard lumps in the breast, hard to the touch, with clear edges. 3. Breast cancer: the cause is still unclear, the typical symptom is the appearance of hard lumps in the breast, which may be accompanied by nipple blood, local pain, orange peel-like skin changes and other symptoms. At the same time, hard lumps in the breast may be associated with breast hyperplasia, chronic inflammation of the breast, ductal papilloma, breast tuberculosis, breast trauma, etc., and need to be carefully identified. There are many causes of hard lumps in the breast of the size of a peanut, build timely to the regular hospital consultation, active treatment.