Idiopathic lumbosacral plexitis (LSP) or non-diabetic lumbosacral radiculoplexus neuropathy (LRPN) is a rare monophasic immune-mediated disorder. The disease is characterized by multiple lumbosacral root and nerve involvement in the absence of trauma, mass lesion or elevated blood sugar. We report a 60-year-old man presenting with a 4-day history of acute-onset sharp left hip pain radiating down his leg associated with numbness over the lateral dorsum of his left foot. MRI of the lumbosacral plexus revealed gadolinium enhancement of mainly L5 on the left. A dramatic response to oral steroids was observed.