Abstract Background. Over the past 20 years, the parental origin of the extra chromosome in children with trisomy 21 has been investigated with cytogenetic methods of identifying morphologic variations in chromosome 21. These studies have concluded that the origin of the extra chromosome 21 was maternal in approximately 80 percent of cases and paternal in about 20 percent. Methods. We studied 200 families, each with a single child with trisomy 21, using DNA polymorphisms as markers to determine the parental origin of the nondisjunction causing the extra chromosome 21. These polymorphisms spanned a region of about 120 centimorgans on the long arm of chromosome 21, from the D21S13 locus (the most centromeric) to the COL6A1 gene (the most telomeric). Results. The parental origin of nondisjunction could be determined for all but 7 of the 200 children. It was maternal in 184 children (proportion [±SE], 95.3±1.5 percent) and paternal in 9 (4.7±1.5 percent). In a subgroup of 31 families, we compared the results ...