The case history is presented of a male infant who was thought to have idiopathic pulmonary arterial hypertension (PAH) at 3 months of age. Subsequently the PAH decreased unexpectedly and diffuse pulmonary arteriovenous malformations (PAVMs) were seen at 6.9 years of age for the first time. Hereditary haemorrhagic telangiectasia type 1 (HHT1) related to an endoglin mutation was diagnosed. At 10.3 years of age a lung biopsy showed diffuse PAVMs as well as pulmonary arteriopathy with medial hypertrophy. This is the first case of HHT1 presenting with PAH at such a young age. The subsequent decrease in pulmonary arterial pressure (PAP) was probably caused by the development of PAVMs. In the presence of PAVMs, measurement of the PAP may underestimate the extent of PAH-related vasculopathy.