P-glycoprotein, a pump located in the plasma cell membrane, extrudes several clinically important drugs from the cell, and hence causes multidrug resistance. Reversing clinical drug resistance is possible by using agents that inhibit the activity of P-glycoprotein. We describe the results of sequential flow cytometric determinations of P-glycoprotein expression and activity in two patients suffering from acute lymphoblastic transformation of chronic myeloid leukaemia. Neither P-glycoprotein expression, nor its activity could be detected in the initial sample of the first patient. In the second patient, no P-glycoprotein expression was found at diagnosis. However, after chemotherapy containing P-glycoprotein substrates, a significant expression was found in both patients and the functional flow cytometric test was positive. In order to achieve an accurate selection of patients that might benefit from the clinical use of P-gp inhibitors, repeated analyses are indicated in each patient suffering from acute leukaemia, during the course of the illness.