The proteasome inhibitor, bortezomib, is known to be effective in the therapy of various neoplasms. The objective of this study was the analysis of the ex vivo activity of bortezomib in paediatric acute lymphoblastic leukaemia (ALL), in comparison to paediatric acute myeloid leukaemia (AML). A total of 159 patients entered the study, including 106 ALL (including 86 precursor-B-cell ALL, and 20 T-cell ALL) and 53 AML children. The ex vivo sensitivity to bortezomib and 16 other drugs was studied by MTT assay. Paediatric AML samples were more resistant than paediatric ALL samples to most of the tested drugs, except for cytarabine and thioguanine. With respect to immunophenotype, ex vivo drug resistance in T-cell ALL (T-ALL) was higher for most of the drugs. No differences in drug resistance between T-ALL and common/pre-B-cell-ALL were found for daunorubicin, mitoxantrone and 6-thioguanine. Bortezomib was the only compound which was more active in T-ALL than in common/pre-B-ALL paediatric samples. In conclusion, bortezomib had good ex vivo activity in paediatric T-ALL samples.