Angioimmunoblastic T-cell lymphoma (AITCL) is a histologically distinct and relatively common subtype of T-cell lymphoma. Although the putative normal cell counterpart is a mature CD4+ T cell, the precise cell of origin remains elusive. We evaluated cases with a diagnosis of AITCL to determine the specificity and utility of CD10 coexpression, particularly by flow cytometry (FCM), in facilitating this diagnosis. Coexpression of BCL6 was also assessed. Eight AITCL cases were evaluated histologically, immunohistochemically, and by 4-color FCM. Four cases of peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS), were also analyzed. The lymphoma cells in all 8 AITCL cases were CD4+, CD45RO+ T cells, with classic extrafollicular meshworks of CD21/CD23/CD35+ follicular dendritic cells. Furthermore, all cases of AITCL cases contained interfollicular CD10+ cells by immunohistochemistry, and increased coexpression of CD10 on T cells was also detected in 6 of 8 cases by FCM. CD10 coexpression was not observed in all 4 PTCL-NOS cases. Although not specific for AITCL, increased numbers of BCL6+ cells were seen in AITCL as compared with PTCL-NOS. Double immunohistochemistry performed on an AITCL case with high numbers of BCL6+ cells highlighted coexpression of BCL6 and CD4 on the same cells. The finding suggests that AITCL may be a neoplasm of (possibly intrafollicular) CD10+, BCL6+, and CD4+ memory T cells. Although our series is small, our results suggest that CD10 coexpression may be a useful discriminant, particularly if the differential diagnosis is PTCL-NOS, and demonstrate that this can be determined by FCM.