The term multidrug resistance (MDR) describes the observation that tumour cell lines can become cross-resistant to several structurally unrelated chemotherapeutic agents after exposure to a single cytotoxic drug. In hematological malignancies, MDR is most often associated with overexpression of P-gp, a 170-kd transmembrane glycoprotein encoded by the human MDRI gene. Indeed, P-gp expression has been correlated with drug sensitivity and clinical outcome in several studies in acute myelogenous leukemia (AML), multiple myeloma (MM), and malignant lymphomas (NHL). A large number of compounds 'off the shelf' have been investigated for their ability to reverse the P-gp mediated MDR. However, most of these agents produced severe toxic effects at doses required to effectively block P-gp function, and modulation of P-gp in normal tissues can affect the pharmacokinetics and, thus, the toxicity of the associated chemotherapeutic agents. Phase I/IIa trials with third generation MDR modulators, such as valspodar, show that these new agents can be safely administered in combination with different chemotherapy regiments after dose adjustments of cytotoxic drugs that a P-gp substrates. Moreover, MDR reversal by valspodar has been demonstrated in the patients with AML, multiple myeloma, and non-Hodgkin's lymphoma. The definition of the clinical benefits of using MDR modulators in haematological malignancies and their full extent awaits the conclusion of the ongoing randomized phase III trials with valspondar in either newly diagnosed or resistant relapsed AML patients, and in multiple myeloma patients who have failed front-line treatment.