A Study to Evaluate the Efficacy and Safety of BMN 111 in Children With Achondroplasia

Inclusion Criteria * Parent(s) or guardian(s) consent * 5 to \< 18 years old * ACH, documented and confirmed by genetic testing * At least a 6-month period of pretreatment growth assessment in Study 111-901 before study entry * If sexually active, willing to use a highly effective method of contraception * Ambulatory and able to stand without assistance Exclusion criteria: * Hypochondroplasia or short stature condition other than ACH * Have any of the following: * Hypothyroidism or hyperthyroidism * Insulin-requiring diabetes mellitus * Autoimmune inflammatory disease * Inflammatory bowel disease * Autonomic neuropathy * History of any of the following: * Renal insufficiency defined as serum creatinine \> 2 mg/dL * Chronic anemia * Baseline systolic blood pressure (BP) \< 70 millimeters of mercury (mm Hg) or recurrent symptomatic hypotension (defined as episodes of low BP generally accompanied by symptoms ie, dizziness, fainting) or recurrent symptomatic orthostatic hypotension * Cardiac or vascular disease * Have a clinically significant finding or arrhythmia on screening electrocardiogram (ECG) that indicates abnormal cardiac function or conduction or Fridericias corrected QTc-F \> 450 msec * Have an unstable condition likely to require surgical intervention during the study (including progressive cervical medullary compression or severe untreated sleep apnea) * Decreased growth velocity (\< 1.5 cm/yr) over a period of 6 months or evidence of growth plate closure (proximal tibia, distal femur) * Treated with growth hormone, insulin-like growth factor 1 (IGF-1), or anabolic steroids in the previous 6 months or treatment greater than 6 months at any time * Greater than 1 month treatment with oral corticosteroids (low-dose ongoing inhaled steroid for asthma, or intranasal steroids, are acceptable) in the previous 12 months * Planned or expected to have limb-lengthening surgery during the study period. Subjects with previous limb- lengthening surgery may enroll if surgery occurred at least 18 months prior to the study and healing is complete without sequelae. * Planned or expected bone-related surgery (ie. surgery involving disruption of bone cortex, excluding tooth extraction), during the study period. Subjects with previous bone-related surgery may enroll if surgery occurred at least 6 months prior to the study and healing is complete without sequelae. * Had a fracture of the long bones or spine within 6 months prior to screening * History of severe untreated sleep apnea * New initiation of sleep apnea treatment (e.g. CPAP or sleep apnea-mitigating surgery) in the previous 2 months prior to screening * History of hip surgery or hip dysplasia atypical for achondroplastic subjects * History of clinically significant hip injury in the 30 days prior to screening * History of slipped capital femoral epiphysis or avascular necrosis of the femoral head * Abnormal findings on baseline clinical hip exam or imaging assessments that are determined to be clinically significant * Concurrent disease or condition that would interfere with study participation or safety evaluations, for any reason * Condition or circumstance that places the subject at high risk for poor treatment compliance or for not completing the study