"Imperforate anus" is a syndrome of anomalies of organization of the caudal end of the body, frequently involving the genitourinary tract and lumbosacral spine, as well as the normal formation of the rectum and anus. In patients with imperforate anus, any type of sacral deformity or error in lumbar segmentation (with subsequent evolution of "block" vertebrae) should be reason for prompt urologic evaluation by intravenous pyelography and voiding cystourethrography in the newborn period. To fail to do so may lead to irreversible damage from reflux. The functional anomaly of reflux may be superimposed on structural malformations including unilateral renal agenesis, and crossed fused and crossed solitary ectopia. A breakdown of the associated anomalies of the "imperforate anus" into high (bowel usually ending above the puborectalis part of the levator ani) and low lesions (bowel ending on the perineum anterior to the anus) shows a much higher incidence of spine and genitourinary changes in cases with the high lesions. The "syndrome of caudal regression" correlates imperforate anus, urologic malformations and lumbosacral defects in development to a common embryologic error of organization in the 5th to 7th week of fetal development.