Hopkins syndrome is a poliomyelitis-like illness manifesting flaccid paralysis of an extremity in the recovery stage after an asthmatic attack. A 7-year-old boy who developed acute flaccid paralysis of the left upper extremity 4 days after an asthmatic attack is reported. T2-weighted magnetic resonance imaging of the cervical spine revealed a local high-intensity area in the left anterior horn at the C4 to C6 level. There have been few pathologic or radiologic studies of this syndrome. We suspect that the cause is an anterior horn lesion.