RESUMEN
A case of a complete lumbosacral agenesis is presented, whose etiological factors are unknown. Clinically, motor deficit corresponded to the agenesis level, but the sensibility and the autonomous nervous system functions, which were conserved, partially, in lower levels than the normal ones in the agenesis. The case report is completed with electromiography, mielography, rectal biopsy, urography, cistography, opaque enema and inborn infection study. There are three facts: a partially conserved sensibility in the lower extremities, normal bladder function and above all, the presence of nervous parasympatics normal fibers in rectum wall, in contrast with the serious motor alteration just in the same medullar levels. Explanation of these facts suggest origin of an embryological source. It is assumed that the noxa must have acted between the third and the fourth weeks of pregnancy at a caudal cordomesoblast level, supposing that it did not affect neural crest.