Robertsonian translocations between acrocentric chromosomes are the most common structural chromosomal rearrangements in humans and many other organisms, and several mechanisms for their formation have been proposed. We have analyzed highly informative DNA polymorphisms in a family with a non-mosaic de novo Robertsonian translocation 21q;21q, to determine the parental origin of the two 21q arms of the rearranged chromosome. The genotypes indicated a biparental origin, i.e. one 21q was paternal and the other maternal. These results imply that in some cases the formation of the rob(21q;21q) occurs in the zygote or in the first few postzygotic mitotic divisions.