Four cases of extrarenal nephrogenic proliferation in the sacrococcygeal re
gion with spinal dysraphism are presented. In two of the cases, features of
Wilm's or incipient Wilm's tumor were present. The previous literature on
sacrococcygeal nephrogenic tissue is reviewed, and the impact of these find
ings on the histogenesis of extrarenal sacrococcygeal Wilm's tumor is discu
ssed.