Patients with myotonic dystrophy (MD), a neuromuscular multisystem disease characterised by progressive muscular weakness, often present with an anterior open bite associated with excessive facial vertical development. These aberrations in vertical craniofacial growth may be due to the reduced function of the masticatory muscles and the lowered position of the tongue. Well-planned and executed treatment of these patients, with orthodontics and orthognathic surgery, can provide satisfactory results. The long-term stability of these treatment results however remains undocumented. This paper describes a case of severe anterior open bite in a patient with MD treated with orthodontics and orthognathic surgery, with an acceptable post-treatment result. Long-term follow-up however revealed a highly unstable outcome. Reasons for this include the continued overeruption of posterior teeth due to the weakened masticatory musculature, mandibular posture, or problems with TMJ adaptation and remodeling. Possible ways to minimise this instability are discussed, namely lifelong skeletal retention or postponing treatment until a later age.