A 16-year-old female track athlete experienced sudden onset of right anterolateral thigh pain, initially thought to be cramping. After 2 months of continued postexercise pain she sought medical evaluation. A 3-cm thigh circumference discrepancy was noted on physical exam. She was referred for electromyography (EMG) and magnetic resonance imaging (MRI) studies. Femoral and screening nerve conduction studies were normal. Needle EMG revealed acute neuropathic changes confined to the right vastus lateralis only. These findings were confirmed by MRI, including short T1 inversion recovery (STIR) sequences. All other medical work-up was normal, including lumbar and pelvic MRI, and complete serologic studies. This case represents a unique presentation of an idiopathic femoral mononeuropathy isolated to the vastus lateralis only, which has not been previously reported. The highly unusual anatomical presentation of this case illustrates the emerging complementary usefulness of EMG and MRI in delineating neuromuscular pathology.