Lumbosacral plexitis (LSP) is an uncommon idiopathic disorder characterized by the acute onset of severe lower extremity pain followed by wasting and weakness of leg muscles with variable sensory loss. Muscles innervated by multiple roots and multiple nerves are affected, and other causes of lumbosacral plexopathy such as mass lesions and trauma must be excluded. Postulated causes of LSP include vasculitis with subsequent axonal injury and autoimmune-related demyelination, although evidence is accumulating that the former pathophysiology is more likely. Recovery is variable but is often quite slow and incomplete. Immunomodulatory therapies such as corticosteroids, intravenous immunoglobulin, and plasma exchange are inconsistent in their efficacy.