Cellular drug resistance is an important determinant of the response to chemotherapy, and its precise measurement may have clinical relevance. Potential applications are: prognostic factor for risk-group stratification, tailored chemotherapy for subgroups or individual patients with a specific cellular drug resistance profile, determination of cross-resistance patterns, study of drug interactions, study of resistance modulation or circumvention, selection of patients for phase II studies and screening for the cytotoxicity of novel compounds. The colorimetric 4-day MTT assay is a frequently used method. However, a distinction between malignant and non-malignant cells cannot be made, which should be taken into account. In the case of a relatively high percentage of contaminating non-malignant cells, the differential staining cytotoxicity (DiSC) assay can be used. The MTT assay's technical success percentage is about 80% for fresh ALL and AML samples. For methotrexate (MTX) a different assay must be used, such as the thymidylate synthase inhibition assay (TSIA). The MTT assay measures the number of living cells that survived drug exposure. Therefore, the effect of many if not most drugs to induce leukemia cell death by apoptosis is also included. This review mainly summarizes the data on cellular drug resistance in childhood leukemia, as obtained by the MTT assay and TSIA, in our laboratory in Amsterdam. These data clearly demonstrate the significant relation between in vitro cellular drug resistance and clinical and cell biological features and short- and long-term clinical outcome in childhood leukemia. In conclusion, cellular drug resistance testing provides clinically relevant information that can be available within 1 week and can be performed successfully in the vast majority of leukemia samples. The data are more and more being used and being considered for use in clinical trials in leukemia.