Pericentric inversions of chromosome number 9 have been studied in 4 different probands: a normal female with designation 46, XX, inv(9)(p12q13); a male with Down syndrome designated as 47, XY,+21, inv(9)(p13q13); a premature infant with multiple, congenital malformations who was 46, XX, inv(9)(p12q21), and a Down syndrome proband with 47, XYqs,+21, inv(9)(p13q21). All 4 cases were shown to be inherited based on family studies. These families are discussed with reference to the literature as to what possible effect this structural change could have on the reproductive capability of a normal carrier and what guidelines are available for counseling such a carrier.