Achondroplasia is the most common skeletal dysplasia in children. Achondroplasic patients have a short cranial face and midface hypoplasia. They often have sleep-related respiratory disturbances that lead to hypoxemia caused by midfacial hypoplasia, a small upper airway, hypotonia of airway muscles, or brain stem compression. It has been well described that obstructive sleep apnea can cause pulmonary hypertension (PH) through the mechanism of chronic hypoxemia. However, severe PH due to obstructive-type sleep disorder is rare in patients with achondroplasia. In this report, we describe a 5-year-old girl with achondroplasia whose severe PH was caused by upper-airway obstruction and was resolved gradually after adenotonsillectomy.