An important subgroup of phyllodes tumors of the breast is characterized by rearrangements of chromosomes 1q and 10q.

From a study of 10 cases of our own and 13 cases of the literature, anomalies of chromosome 1q and 10q emerge as consistently occurring changes in an important subgroup of phyllodes tumors of the breast. Anomalies of chromosome 1 were the most frequent ones, with a gain of 1q material, and histologically the tumors in which these anomalies were found were low grade malignancies. Structural changes of 10q emerged as the second most frequent chromosome change.