Pulmonary arterial hypertension (PAH) is a rare disease in which high blood pressure in vessels of the lung leads to heart failure and premature death. PAH is caused by various genetic and environmental factors, including recently identified autosomal dominant ATP13A3 variants causing adult-onset PAH. We report three families with a novel, autosomal recessive form of severe childhood-onset PAH due to two variant copies of ATP13A3. The children were largely resistant to treatment and four died in early childhood. The data are consistent with a dose-dependent effect of ATP13A3 variant copy number in which two variant copies cause severe, early-onset PAH. (By Dr. Carrie Welch, https://jmg.bmj.com/content/early/2021/09/07/jmedgenet-2021-107831 )