We have cytogenetically investigated a total of 33 simple benign endometrial polyps, 7 of which have been reported previously. Clonal chromosome rearrangements are found in 19 of 33 lesions (57%). Three major cytogenetically abnormal subgroups can be distinguished: (a) those with rearrangements in the 6p21-p22 region; (b) those with rearrangements of the 12q13-15 region; (c) those with rearrangements of the 7q22 region. A normal karyotype is found in a fourth subgroup. Recombinations of the 6p21-22 region with 2q35 and 10q22, as well as rearrangements of 7q22, have not been described before. It can be concluded that endometrial polyps, like several other types of benign mesenchymal tumors, present several cytogenetically different subgroups despite a seemingly identical clinical and morphological appearance. It is mandatory, therefore, to look for a common denominator of these tumors at the molecular level.