Sixteen triploid and one tetraploid human abortuses were studied for the origin of polyploidy using a sequential Q- and R-banding technique. Ten triploid abortuses provided informative data: one originated in the maternal first meiotic division; five apparently resulted from dispermy; two derived from an error during either the paternal second meiotic division or the first mitotic division; and the last two were of paternal origin. The results indicate that paternal factors, especially dispermy, are the predominant sources of triploidy in man. The tetraploid abortus showed duplication of both the maternal and paternal haploid sets, suggesting normal division of chromosomes and suppression of cytoplasmic cleavage at the first mitotic division. No correlation was found between the origin of polyploidy and the phenotype of the triploid abortuses, nor between the origin of polyploidy and the maternal use of oral contraceptives.