There are many reasons for acne behind the head, such as folliculitis, acne, seborrheic dermatitis and so on. 1. Folliculitis: If you don’t pay attention to local hygiene, dandruff, oil and other residues on the scalp, it is easy to harbor pathogens in the long run, and if the bacteria or fungus infects the local hair follicles, folliculitis is triggered, resulting in the phenomenon of pimples on the back of the head. 2. Acne: Acne is a multifactorial disease, mainly associated with changes in hormone levels in the body, a large amount of secretion of cortex, abnormal keratinization of the cells around the mouth of the hair follicle, Propionibacterium acnes infections and other factors, especially in adolescents, if the back of the head of the skin excessive secretion of sebum, the keratinization of the cells around the mouth of the hair follicle is too much, and infection with Propionibacterium acnes at the same time, then the phenomenon of pimples will be caused. 3. Seborrheic dermatitis: the cause of this disease is not yet clear, currently considered to be related to genetic factors, long-term late night, improper diet, mental stress, abnormal secretion of oil, microbial infections, etc., mainly manifested in the back of the head of the greasy erythema, scales, pimples, which can be accompanied by itching, which will appear behind the head of the phenomenon of long acne. The causes of acne on the back of the head are very many, it is recommended that patients with this symptom go to the hospital in a timely manner to improve the examination, identify the causes and take standardized treatment, do not ignore the symptoms, so as not to delay the diagnosis and treatment of the condition.