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. T-2-weighted magnetic resonance imaging of the cervical spine revealed a local high-intensity area in the left anterior horn at the C-4 to C-6 level. There have been few pathologic or radiologic studies of this syndrome. We suspect that the cause is an anterior horn lesion.