The transcription factor PAX5 plays a key role in the commitment of hematopoietic precursors to the B-cell lineage, but its expression in acute leukemias has not been thoroughly investigated. Hereby, we analyzed routine biopsies from 360 acute leukemias of lymphoid (ALLs) and myeloid (AMLs) origin with a specific anti-PAX5 monoclonal antibody. Blasts from 150 B-cell ALLs showed strong PAX5 nuclear expression, paralleling that of CD79a in the cytoplasm. Conversely, PAX5 was not detected in 50 T-cell ALLs, including 20 cases aberrantly coexpressing CD79a. Among 160 cytogenetically/molecularly characterized AMLs, PAX5 was selectively detected in 15 of 42 cases bearing the t(8;21)/AML1-ETO rearrangement. Real-time reverse transcription-PCR studies in t(8;21)-AML showed a similar up-regulation of PAX5 transcript in all of the 8 tested samples (including 4 cases that were negative at anti-PAX5 immunostaining), suggesting that PAX5 is expressed in t(8;21)-AML more widely than shown by immunohistochemistry. Interestingly, PAX5(+) t(8;21)-AML also expressed CD79a and/or CD19 (major transcriptional targets of PAX5 in B-cells) in 10 of 12 evaluable cases. Our results indicate that PAX5 is a more specific marker than CD79a for B-cell ALL diagnosis. Moreover, among AMLs, PAX5 expression selectively clusters with t(8;21), allowing its immunohistochemical recognition in a proportion of cases, and likely explaining a peculiar biological feature of this subset of myeloid leukemias, i.e. the aberrant expression of B-cell genes.