Sweating in the armpits can be due to suppurative sweating, folliculitis, trauma and other causes. 1. Purulent Sweatitis: It is a chronic purulent inflammation of the parietal sweat glands, also known as paradoxical acne. The cause of the disease may be related to genetic factors, immune disorders, smoking, obesity and other factors. The patient’s lesions may initially appear as small nodules, and as the disease progresses may become pus-filled and ulcerated. When the armpit sweating, sweat stained the affected area, can appear painful. 2. Folliculitis: mainly caused by bacterial, fungal and other pathogenic bacteria infection. It is a red papule in the early stage, which rapidly develops into papular pustules. As sweat contains salt, it can irritate the wound and painful sensation occurs. 3. Trauma: It may also be due to the patient shaving the armpit hair, accidentally scrape the armpit wound. The sweat component stimulates the wound and the pain appears. It is recommended that patients whose armpits hurt when they sweat go to the hospital in time and follow the doctor’s instructions for treatment to avoid delaying their condition.