Elevated homocysteine (Hcy) plasma levels are caused by genetic and environmental factors. Polymorphisms of Hcy metabolizing enzyme genes may result in its plasma increase. Experimental and clinical studies have shown the possible role of hyperhomocysteinaemia in pathogenesis of Parkinson's disease (PD), Alzheimer's disease and vascular disorders. The results of clinical studies in PD generally do not support the theoretical hypotheses, and animal studies remain controversial. A major environmental factor responsible for Hcy increase in PD seems to be levodopa therapy. Its metabolism results in Hcy increase and may be reduced with folate and vitamins B6, B12 supplementation or inhibition of catechol-O-methyltransferase (COMT) activity. Therefore, the potential harmful role of Hcy may be diminished in PD patients with vascular comorbidities. Further studies are needed to establish the real role of Hcy for PD and other neurological disorders. The paper summarizes the current knowledge on the genetic and environmental factors responsible for Hcy increase in PD.