A previously healthy 4-year-old boy presented with left arm weakness and pain that had lasted 1 day. He had no history of trauma, headache, tick bites, or numbness or tingling in his extremities. The child had had coryza 1 week earlier, which self-resolved. On physical examination, the child appeared well and had normal vital signs. Neurologic examination demonstrated 3 out of 5 strength of the left shoulder girdle muscles and 4 out of 5 strength of the left lower arm, with normal sensation. He had decreased strength with abduction and external rotation of the left shoulder. The patient could not elevate the left arm past 80° (Figure 1and Figure 2). The results of the remainder of his physical examination were normal.