Hard pimples under the armpits may be caused by folliculitis, sebaceous cysts, enlarged lymph nodes and so on. 1. Folliculitis: the distribution of hair follicles in the armpit is exuberant, and the secondary infection of hair follicles may lead to protruding hair follicles, swelling, pain, etc., which may cause hard acne in the armpit. 2. Sebaceous cysts: If the sebaceous gland ducts are blocked, the sebaceous gland secretions cannot be discharged in time, and the sebum will be retained to form a cystic swelling, which will lead to the phenomenon of hard acne under the armpit. If secondary bacterial infection, it will cause skin redness, swelling, pain and other symptoms. 3. Enlarged lymph nodes: when the lymph nodes are attacked by pathogenic microorganisms or tumors, the lymph nodes will be enlarged, resulting in the phenomenon of hard acne underarms, which may be accompanied by symptoms of swelling and pain. Patients with hard acne underarms, should immediately go to the hospital, clear the cause of the disease under the guidance of the physician standardized treatment.