A hard lump in the cleavage is usually caused by sebaceous cysts, breast hyperplasia and lipoma. 1. Sebaceous cysts: excessive secretion of sebaceous glands triggers unsmooth drainage holes, leading to sebaceous gland drainage obstruction and thus leading to sebaceous cysts, usually appearing in the cleavage with hard lumps, touching the elasticity of the skin color blue and other symptoms. 2. Breast hyperplasia: it may be caused by the patient’s endocrine disorders leading to breast hyperplasia, usually there will be breast pain, nipple discharge, hard lumps in the cleavage and other symptoms. 3. Lipoma: It is usually caused by the patient consuming more greasy food, which leads to abnormal fat metabolism, resulting in fat deposition under the skin, usually with symptoms such as localized swelling and pain, or infection, and hard lumps in the cleavage may also occur. A hard lump in the cleavage may also be caused by breast trauma, breast tumors, etc. It is recommended that patients with a hard lump in the cleavage go to the hospital for timely examination and treatment under the guidance of the doctor.