We describe 10 cases of lumbosacral plexus neuropathy in which no underlying condition was discovered on initial evaluation or on follow-up examination after an average of 6 years. The patients presented with pain and weakness. Recovery was delayed and often incomplete. When the lower plexus is involved, it may be confused with disk disease manifesting as “sciatica.” This syndrome may be a counterpart to the well-described idiopathic brachial plexus neuropathy.