Hallus valgus should be regarded as symptom/sign rather than a diagnosis, because its development may be complicated by congenital joint laxity (ex. Marfan’s syndrome), ill-fitting foot and obesity etc. A 18-year old male with unilateral hallus valgus was presented. With thorough physical examination, the diagnosis of Marfan’s syndrome is confrirmed. Clinical manifestation, diagnosis and surgical management of this case are discussed.