Acne on the outside of the thighs is considered to be caused by folliculitis, keratosis pilaris, eczema and other diseases. 1. Folliculitis: the cause of folliculitis is mainly caused by bacteria, fungi and other pathogens infecting the hair follicles, triggers include skin trauma, hair pulling, wearing tight clothing, etc., the patient will appear to the hair follicle as the center of the red pimples, may be itchy, tenderness, and other symptoms, which can be seen in all parts of the body. 2. Keratosis Pilaris: Keratosis Pilaris is a chronic follicular keratosis skin disease, the cause of which is related to genetics, vitamin A deficiency, endocrine factors, often occurring in the upper arms, thighs, etc., and localized follicular papules of the size of a needle tip to a grain of corn. 3. Eczema: eczema is a chronic inflammatory itchy skin disease, polymorphic rash, symmetrical distribution, can appear on both sides of the thighs, the patient will be conscious of itching, the cause of the disease and genetics, immune abnormalities and so on. There are more reasons for acne on the outer thighs, so it is recommended to consult a doctor for a clear diagnosis.