During the mid sixties scientists recognized that tumour cells can be resistant to a variety of chemotherapeutical drugs of different chemical structure simultaneously. They named this phenomenon multidrug resistance (MDR). Following this observation, number of in vitro and in vivo experiments proved that transmembrane proteins of the cell membrane are responsible for the mechanism. Many details of the underlying biochemical mechanisms were explored during the past decade. Nowadays the importance of MDR is well appreciated in different walks of medical science. MDR is an important problem during the treatment of many haematological conditions and solid organ tumors. Also, MDR is an important factor during immunosuppressant therapy of the transplanted patients. In spite of extensive research there are many uncertainties around MDR. This brief review describes the present options in the investigation of MDR. Based upon the MDR genotyping and expression level the likelihood of drug resistance may be predicted with reasonable accuracy. Additional information may be obtained by measuring the P-glycoprotein expression on the cell surface and the outward transport of test molecules from the cells. Although the tests described above provide significant help in predicting MDR or in the confirmation of existing MDR there is no consensus about the laboratory diagnosis.