We have coined the term "pseudo-trisomy 13 syndrome" to designate cases of holoprosencephaly, severe facial anomalies, postaxial polydactyly, various other congenital defects, and normal chromosomes. Eleven instances are summarized. Two pairs of sibs and two other cases with consanguinity suggest autosomal recessive inheritance. Autosomal recessive inheritance is possible. Alternately, an undetected microdeletion and etiologic heterogeneity (some cases possibly representing dominant new mutations) must be considered. Further delineation is necessary. It is hoped that this paper will serve as a focus for further discussion of the problem.