Sudden red patches on the face may be caused by eczema, contact dermatitis, seborrheic dermatitis and other diseases. 1. Eczema: the cause of eczema is not yet clear, may be related to heredity, immune abnormalities and hormone level changes, etc. It occurs in the head, face and other parts of the body, localized erythema, papules, pimples and other symptoms. 2. Contact dermatitis: contact dermatitis is characterized by erythema, swelling, pimples, blisters, etc. It is a disease caused by skin contact with external stimulating substances, and can appear on the face. 3. Seborrheic dermatitis: seborrheic dermatitis is commonly found on the scalp, face and other parts of the body, the cause of which is not yet clear, and may be related to heredity, sebum metabolism and so on. Patients will have localized erythematous rash, flaking, itching and other symptoms. There are more causes of sudden red patches on the face, so it is recommended to consult a doctor for examination and clear diagnosis.