A 22-year-old male student had a 7-year history of intermittent painful sensation in the right Achilles tendon and the posterior part of the leg and thigh that followed a distribution suggestive of an L5 or S1 radiculopathy. For 2 years, he has noticed some weakness in his right leg, with footdrop and limping while walking. When he came to our attention in 2008, the neurologic examination demonstrated diffuse atrophy of the right lower limb (Figure, A), without fasciculation. Mild paresis of knee flexion and of foot and toe dorsiflexion was found, with partially reducible right footdrop. Knee jerk was diminished, and ankle jerk was abolished. No cutaneous sensory deficit could be delineated. An electromyogram demonstrated sensorimotor axonal loss in the territory of the sciatic nerve, more marked in its tibial branch, where 75% axonotmesis of motor fibers and 50% axonotmesis of sensory fibers were found. A magnetic resonance image of the right thigh showed a gigantic, lobulated tumor entrapping the sciatic nerve and extending from the greater sciatic foramen to the distal tier of the thigh (Figure, B).