By using RNA slot-blot technique, the frequency and the degree of GST pi and mdr-1 gene coexpression were investigated in 23 AML patients, 9 ALL, 9 CLL and 11 cases of NHL in an attempt to study their clinical and prognostic relevance. GST pi and mdr-1 levels were expressed as arbitrary units (U) with respect to the negative controls (U = 0), MCF7 and HL60 sensitive cell lines, and the positive controls (U = 10), MCF7/DOXO and HL60/DNR resistant cell lines. The concomitant GST pi/mdr-1 gene overexpression showed a negative prognostic value in the set of newly diagnosed AML pts (10 cases), furthermore higher GST pi and mdr-1 mRNA levels were averagely detected in the relapsed/resistant ALL pts (4 cases), and in CLL (7 cases) and NHL (8 cases) heavily pretreated patients who were unresponsive to chemotherapy and with a disease progression. These preliminary data show that two different mechanisms of drug resistance can be coexpressed at the same time in those leukemias and lymphomas with a clinically unfavourable course.