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Ischaemic strokes in young adults carry considerable mortality and morbidity; however, their cause is often unknown. A 25-year-old man experienced sudden-onset right homonymous hemianopia after 1 week of gradually worsening left-sided neck pain; imaging confirmed left vertebral artery dissection with associated posterior cerebral circulation infarction. Ten days later, he experienced right-sided neck pain consistent with a right vertebral artery dissection, but with no recent trauma. Genetic testing identified a novel heterozygous variant in SMAD3 (Mothers against decapentaplegic homolog 3), which is associated with Loeys–Dietz syndrome 3. This case highlights the importance of comprehensive diagnostic investigations for young adults with ischaemic stroke, including consideration of genetic testing. Accurately identifying genetic causes of stroke allows improved patient management including familial screening, clinical surveillance and pre-implantation genetic screening.