The parental origin of the extra chromosome 21 (or extra 21q) was determined in seven informative families with a Down syndrome (DS) child by using molecular polymorphisms. Five DS patients had regular trisomy, one a de novo 14/21 translocation and another a de novo 21/21 translocation or isochromosome 21q. In four families with regular trisomy, the extra chromosome was of maternal origin, and in one family it was paternally derived. In the two families with a de novo aberration, both the 14/21 translocation and 21/21 rearrangement originated during maternal meiosis. For a better evaluation of the stage of meiotic error and the occurrence of crossovers between nondisjoined chromosomes, the regional map position of four of the nine informative DNA markers, used in this study, was refined, leading to useful localizations in both centromeric and distal regions. Recombination events were found in two families with regular trisomy, one occurring between chromosomes 21 that failed to disjoin at maternal meiosis I, the other prior to a paternal meiosis II nondisjunction.