At relapse, T-cell acute lymphoblastic leukaemia (ALL) has a worse patient outcome than B-cell precursor (BCP-) ALL. To investigate this further, we compared in vitro cellular drug resistance profiles of T-cell and BCP-ALL samples obtained at relapse. We investigated 237 paediatric relapsed ALL cases, including 151 samples taken at first relapse, of which 30 were T-cell ALL. In vitro drug resistance was measured using the 4-day methyl-thiazol-tetrazolium (MTT) assay and cellular immunophenotype was determined at central reference laboratories. Similar results were found for first relapsed ALL samples and for the total group: T-cell ALL samples were more resistant to 4-HOO-ifosfamide (1.4-fold, P = 0.019) and cisplatin (3.7-fold, P = 0.005). The samples were more sensitive to thiopurines such as mercaptopurine (2.1-fold, P = 0.007) and thioguanine (1.7-fold, P = 0.003). Resistance/sensitivity to 16 other drugs did not differ significantly. These results do not explain the relatively poor prognosis of T-cell ALL at relapse, but do suggest that the more intensive use of thiopurines in relapsed T-cell ALL may be beneficial.