Suprascapular nerve palsy resulting in isolated weakness of the infraspinatus muscle is subtle at its onset and must be differentiated from cervical radiculopathy or bone and joint diseases of the shoulder, especially in the presence of pain. The literature suggests an association of cumulative trauma with ganglionic cyst formation and entrapment neuropathy of the suprascapular nerve in the spinoglenoid notch. Here we present clinical, electrodiagnostic, and radiologic features in a young patient who presented with a 2-week history of isolated, painless weakness of the right shoulder. His electromyography showed acute denervation of the right infraspinatus muscle. Magnetic resonance imaging revealed a ganglionic cyst in the spinoglenoid fossa and edema in the infraspinatus muscle consistent with denervation. The patient recovered after removal of the cyst.