Red and painful underarms may be related to folliculitis, sweat, lymphadenitis and other causes, which need to be further differential diagnosis by the doctor. 1. Folliculitis: Bacterial infection of the hair follicles in the underarm skin can cause a localized inflammatory reaction, resulting in redness, swelling and pain in the underarms. 2. Sweatitis: If the bacteria infect the sweat glands in the armpit area, it can cause sweatitis, resulting in redness and pain in the armpit. 3. Lymphadenitis: acute or chronic inflammation of the drainage area of the axillary lymph nodes involves the lymph nodes and causes non-specific inflammation, resulting in localized redness, swelling, heat and pain. It is recommended that people with red and painful underarms go to the hospital in time to clarify the cause of the disease and carry out standardized treatment under the guidance of the doctor to avoid delaying the condition.