Some reports have emerged describing the occurrence of Guillain-Barré syndrome and polyneuropathy related to vitamin B(12) deficiency in some patients with Parkinson's disease (PD) treated with continuous duodenal levodopa infusion. We describe five PD patients who developed axonal polyneuropathy and vitamin B(12) deficiency while on treatment with duodenal levodopa infusion, review other cases reported in the literature, discuss potential etiologic factors, and suggest a possible algorithm for the management and prevention of this complication. One case of Guillain-Barré syndrome and at least 12 cases of polyneuropathy related to vitamin B(12) deficiency have been reported in PD patients treated with duodenal levodopa infusion. Levodopa gel infusion may induce a decrease in vitamin B(12) levels, leading to peripheral neuropathy. Additional pathogenetic mechanisms include alterations related to the metabolism of L: -dopa, abnormal L: -dopa absorption, and direct neurotoxicity of L: -dopa at high doses. Vitamin B(12) supplementation may need to be considered in PD patients on duodenal levodopa infusion therapy. Vitamin B(12) deficiency in patients on duodenal levodopa infusion therapy may be more frequent than the published data suggest. We must be alert.