The long-term safety and efficacy of using induced pluripotent stem cell (iPSC)-derived neurons to replace midbrain dopaminergic neurons lost in Parkinson disease (PD) have not been tested in primates. Kikuchi et al. grafted human iPSC-derived midbrain dopaminergic progenitors into the putamen of macaques that had been treated with the toxin MPTP (which ablates nigral dopaminergic neurons). Three or four monkeys per group received vehicle or cells derived from healthy controls or individuals with PD. Cell transplants markedly improved neurological and movement scores at 12 months, and imaging and histological analyses confirmed that the grafts were tumour-free after as long as 24 months, supporting the utility and safety of this cell-based therapy.