We report a baby born with three fully developed normally functioning legs. The third leg was attached to the baby's sacrum via an iliac bone at the site of a spina bifida. A primitive perineum was present. Innervation of the limb was via a sacral hiatus with a myelomeningocele. A rudimentary peritoneal sac was found at operation containing a blind loop of bowel suspended by a mesentery. The additional leg was found to have no cortical representation using somatic evoked potentials although lumbar responses were normal. Magnetic resonance imaging demonstrated a bifid spinal cord in the lumbo-sacral region. Associated abnormalities were a haemangioma of the L orbit and a vascular mass in the cerebellum.