Autonomic dysreflexia is a condition that develops after spinal cord injury in which potentially life-threatening episodic hypertension is triggered by stimulation of sensory nerves in the body below the site of injury. Central sprouting of small-diameter primary afferent fibers in the dorsal horn of the spinal cord occurs concurrently with the development of this condition. We propose a model for the development of autonomic dysreflexia in which increased nerve growth factor (NGF) in the injured cord stimulates small-diameter primary afferent fiber sprouting, thereby magnifying spinal sympathetic reflexes and promoting dysreflexia. We identified this population of afferent neurons using immunocytochemistry for calcitonin gene-related peptide. Blocking intraspinal NGF with an intrathecally-delivered neutralizing antibody to NGF prevented small-diameter afferent sprouting in rats 2 weeks after a high thoracic spinal cord transection. In the same rats, this anti-NGF antibody treatment significantly decreased (by 43%) the hypertension induced by colon stimulation. The extent of small-diameter afferent sprouting after cord transection correlated significantly with the magnitude of increases in arterial pressure during the autonomic dysreflexia. Neutralizing NGF in the spinal cord is a promising strategy to minimize the life-threatening autonomic dysreflexia that develops after spinal cord injury.