Autonomic nervous system dysfunction occurs rarely after botulinum toxin type A (BTX-A) intramuscular injections. We report a case of a 23-year-old man with spastic diplegia who had transient erectile dysfunction after intramuscular injection of BTX-A (total dosage, 300 IU, body weight 95 kg) in both hamstring muscles. Some investigators believe that the local spread of the toxin is responsible for autonomic dysfunction, while others believe that the transportation of the toxin to the spinal cord via retrograde flow or via the blood flow after entering the circulation are possible mechanisms of neurologic side effects. On the basis of our case, a retrograde axoplasmic flow to the spinal cord could probably occur because the spinal cord level of hamstring muscles is close to spinal cord levels responsible for erection control.