Chromosomes from 39 cases of benign uterine leiomyomas were studied. Consistent chromosomal abnormalities were detected in 15 cases (38.5%). Abnormalities involving chromosomes 12 and 14 with or without additional chromosomal changes were found in five cases (12.8%). Deletion of chromosome 7 was detected in five cases; in three cases (7.6%), this was the only abnormality present. Complex translocations involving X, 5, and 14 as well as X, 3, and 14 were observed in one case each. Insertion of a portion of chromosome 4 to chromosome 1, deletion involving chromosome 3, and nonreciprocal translocation between chromosomes 14 and 15 were observed in one case each. Monosomy 22, with a derived chromosome 14, was observed in one case. Trisomy 7 was also identified in one case. The structural and numeric abnormalities involved chromosomes X, 1, 2, 3, 4, 5, 6, 7, 9, 10, 12, 13, 14, 15, and 22. A normal 46,XX stem line with one or two abnormal cells was observed in 20 cases. Only normal karyotypes were obtained in the remaining four cases. A review of the literature and the results of our study indicate that uterine leiomyomas may be divided into eight groups based on cytogenetic analysis.