A 22-year-old man presented for orthodontic surgery because of mandibular prognathism. Clinical symptoms suggested acromegaly, and diagnosis was verified by an endocrinologist as well as by radiograph. Bilateral mandibular prognathism often represents the first and most striking physical characteristic of acromegaly; usually, it is also the main reason why patients seek help from orthodontists or maxillo-facial surgeons. This case report recapitulates the clinical and histopathological findings in pituitary growth hormone (GH) adenomas and emphasises their importance in surgical orthodontic planning. Mandibular prognatism, macroglossia and abnormal growth of hands and feet represent strong indicators for the diagnosis of acromegaly. This disease and its complications not only affect the entire body but increase mortality if the pituitary gland tumour remains untreated.