Summary. In a series of 60 ALL samples drawn during different stages of the disease we used a cDNA-PCR approach to analyse the relative mRNA levels of the MDR-associated genes encoding mdrl/P-glycoprotein, mrp, and the topoisomerase II isozymes α and β. Expression analysis of the cyclin A gene was included to examine cellular proliferation activity. The expression of gapdh served as an internal standard. Calculating the mean values we found: (i) a distinctly lower mdrl gene expression in primary ALL and first relapses compared to bone marrow from healthy donors, (ii) no change in mdrl and mrp, but a decreased topoisomerase IIα gene expression in first relapses of ALL compared to the primary leukaemia, and (iii) increased mdrl and mrp levels combined to decreased topoisomerase IIα levels in recurrent relapses of ALL showing significant correlations (mdrl/mrp: rs=+0.6833, P<0.05: mdrl/topollα: rs− 0.6727, P < 0.05). The expression of the topoisomerase IIá gene was correlated to that of cyclin A, indicating a link of its expression to cellular proliferation. Our findings suggest that a multifactorial MDR including mrp appears particularly in recurrent relapses of ALL. which often do not respond to chemotherapy. Nonetheless, some individual samples showed gene expression levels very different from the mean values calculated for a particular state of the leukaemia, indicating the need of an individual expression analysis of MDR-associated genes.