Giant cell-rich bone lesions consist of tumor-like lesions and true neoplastic giant cell tumors. In this study it was investigated whether DNA cytometry may contribute to the differential diagnosis between aneurysmal bone cysts and giant cell tumors. Statistically significant differences in the frequency of tetraploid stemlines were detected. Nevertheless, the knowledge of age, localization and radiological signs in addition to morphological findings are essential to distinguish between these lesions.