Medial aplasia of the spine is a rarely recognized malformation characterized by isolated absence of spinal segments, with vertebrae present cranially and caudally. We analyzed six patients and the five cases described in the relevant literature. The extent of defect ranges from one to 11 segments and leads to spinal instability with resulting kyphosis. All patients have a thoracic or high lumbar level of paralysis and, in varying degrees, typical contractures of hips, knees, and feet. The differentiation between medial aplasia of the spine and the syndrome of caudal regression is discussed.