Previous reports of primary lumbosacral plexopathy (LSP) have stressed the benign nature of the syndrome. No effective treatment is known for the occasional patients who do not recover or who have relapses. We report two patients presenting with a progressive form of idiopathic LSP. Both patients showed remarkable improvement on high-dose intravenous immunoglobulin therapy. We suggest that these patients represent a treatable subgroup of LSP.