Brown spots in the armpits are painless and itchy may be caused by diseases such as florid chaff rash, seborrheic keratosis, and post-inflammatory hyperpigmentation. 1. maculopapular furuncle: the etiology is mostly related to Malassezia infection, genetic factors, environmental factors and other reasons may also induce the disease. Patients will appear localized clear boundary rash, usually brown, light red, etc., the center can subside as a wreath, can be seen in the chest, upper back, armpits and other parts of the body, usually does not produce self-conscious symptoms. 2. Seborrheic keratosis: the cause of the disease may be related to skin aging, genetic mutation, heredity and other factors. Patients will have light brown to black patches, which can be seen on any part of the body except palms, soles and oral mucosa, and usually do not have any discomfort. 3. Post-inflammatory hyperpigmentation: the cause may be related to the environment, trauma and other factors. The skin where inflammation occurs in patients will appear dark brown, light brown patches or patches, usually without conscious symptoms, if the armpit has ever had inflammation, leaving pigmentation in the late stage, there may be brown spots. If brown spots appear in the armpits and are not painful or itchy, it is recommended to go to a regular hospital for examination to clarify the cause.