Hemispheric asymmetry in neuronal processes is a fundamental feature of the human brain and drives symptom lateralization in Parkinson's disease (PD), but its molecular determinants are unknown. Here, we determine epigenetic changes and genes involved in hemispheric asymmetry in the healthy and PD brain. Neurons of healthy individuals exhibit numerous hemispheric differences in DNA methylation, affecting genes implicated in neurodegenerative diseases. In PD patients, hemispheric asymmetry in DNA methylation is even greater and involves many PD risk genes. Moreover, the lateralization of clinical PD symptoms involves epigenetic, transcriptional, and proteomic differences across hemispheres that affect neurodevelopment, immune activation, and synaptic transmission. In aging, healthy neurons demonstrate a progressive loss of hemisphere asymmetry in epigenomes that is amplified in PD. For PD patients, a long disease course is associated with retaining more hemispheric asymmetry in neuronal epigenomes. Hemispheric differences in epigenetic gene regulation are prevalent in neurons and may affect the progression and symptoms of PD.