A disturbance of the X chromosome inactivation process has been demonstrated in some human tumors. In order to study this phenomenon in leukemia, the site of unusually frequent folding in Xq13-q21 was used as a morphological marker. A total of 858 GTG-banded mitoses from 61 women with different hematological malignancies were analyzed, 35 of them with normal karyotype and 26 with chromosome aberrations. The folding patterns and the fold percentage remained unchanged in our patients. Using this marker, X-inactivation could easily be verified in leukemia and related disorders on routine preparations.