Exstrophy of the bladder (EB) and exstrophy of the cloaca (EC) are generally recognizable as distinct clinical entities. In patients with EB, the posterior bladder wall is exposed through a midline defect of the abdomen. The umbilicus is inferiorly displaced and located close to the superior margin of the exstrophic bladder. Genital abnormalities are common in boys and girls who may present epispadias and a small, split phallus or a split clitoris, a bi®d uterus, and a duplicate or exstrophic vagina. In contrast to classic EB, EC is commonly associated with omphalocele, spinal defects, and incompletely formed external genitalia and is always associated with imperforate anus. Some authors state that EC and EB constitute two distinct disorders, but others consider them part of a ``continuum,'' representing different levels of severity within the same spectrum. The use of the acronym OEIS to refer to the combination of omphalocele, exstrophy, imperforate anus, and spinal defects, in our opinion, has not helped to clarify the clinical de®nition, pathogenesis, or cause of this multiple congenital anomaly (MCA) pattern, mostly because the term makes no distinction between EC or EB. Here we present the epidemiological analysis of a group of characteristics in infants with EC and infants with EB to determine if they constitute two different entities. We also analyze if the different combinations of omphalocele, imperforate anus, and spinal defects are more frequent in infants with EC than in infants with MCA patterns other than EC and EB. The prevalence in our data for EC was 1:200,233 live births and 1:35,597 for EB. The clinical analysis indicated that the study defects (omphalocele, spine defects, spina bi®da, and imperforate anus) tend to occur together in the same child with a higher frequency if the child has the EC defect than in infants with MCA patterns that did not include EC or EB. Our ®ndings of low birth weight, twinning, single umbilical artery, and preferentially associated malformations suggest that EC is the result of damage occurring very early in development and that EC and EB are two different expressions of a primary polytopic developmental ®eld defect.