Brown patches under the armpits may be due to hyperpigmentation, or diseases such as florid chaff rash and acanthosis nigricans. 1. Hyperpigmentation: hereditary, endocrine such as pregnancy, physical such as sunlight exposure, chemical factors such as drugs or chemicals, post-inflammatory and other factors. Pathogenesis includes the increase in the number of melanocytes or their activity, which ultimately leads to an increase in skin pigmentation and deepening of the color, and may appear in the case of underarm long brown patches. 2. Florid chaff rash: the causative agent of the disease is Malassezia, and the common risk factors for the disease include heredity, humid or high temperature environment, excessive secretion of skin oil, and frequent wearing of non-breathable clothes, and so on. The main symptom is the presence of pitting rash on the proximal upper limbs, neck, chest, etc. The color can be brown, light brown or yellow, light white, etc. It can be manifested as long brown patches on the armpits. 3. Acanthosis nigricans: the etiology of this disease is not clear, but it is considered to be related to factors such as chromosomal dominant inheritance, autoimmune deficiency, obesity and hormonal drugs. It often occurs in the folds of the skin, so the patients may have the performance of long brown patches in the armpits. It is recommended that patients with long brown patches in the armpits go to the hospital to clarify the specific cause.