Graft‐induced dyskinesias are a serious complication after neural transplantation in Parkinson's disease. One patient with Parkinson's disease, treated with fetal grafts 14 years ago and deep brain stimulation 6 years ago, showed marked improvement of motor symptoms but continued to suffer from OFF‐medication graft‐induced dyskinesias. The patient received a series of clinical and imaging assessments. Positron emission tomography and single‐photon emission computed tomography 14 years posttransplantation revealed an elevated serotonin/dopamine transporter ratio in the grafted striatum compatible with serotonergic hyperinnervation. Inhibition of serotonin neuron activity by systemic administration of a 5‐HT_1A agonist suppressed graft‐induced dyskinesias. Our data provide further evidence that serotonergic neurons mediate graft‐induced dyskinesias in Parkinson's disease. Achieving a normal striatal serotonin/dopamine transporter ratio following transplantation of fetal tissue or stem cells should be necessary to avoid the development of graft‐induced dyskinesias. © 2011 Movement Disorder Society