Indolent T lymphoblastic proliferations have been reported rarely in extramedullary and extrathymic tissues. Recent work has identified these indolent T lymphoblast populations to be mostly CD4+/CD8+ (characterized by immunohistochemistry), with only limited immunophenotypic evaluation of these populations by flow cytometry (FC). We retrospectively reviewed our institutional FC archives and identified 12 samples from 10 patients with incidental T lymphoblastic populations. Samples were characterized with respect to expression of T-cell antigens, CD45, TdT, CD1a, and T/NK antigens, and light scatter properties. Overall, the proportion of T lymphoblasts was small (range 0.01-8.8% of white cells; mean, 1.7%). Histologic correlation showed scattered immature T lymphoblasts in samples without overt distortion of underlying architecture. T lymphoblasts were identified most frequently in association with Castleman disease (four) or tissues with Castleman features (four), marginal zone B-cell lymphoma (one), or tissue with reactive/atypical changes (three cases). Although three cases were composed predominantly of CD4+/CD8+ T cells, the majority of cases in our cohort (eight) included a major subset of CD4-/CD8- T lymphoblasts by FC (one case CD8+/CD4-), which has not been described previously. There was no evidence of subsequent progression to T lymphoblastic leukemia. Incidental, indolent T lymphoblastic proliferations may be detected by clinical FC. In contrast to reports, we find that these proliferations in clinical samples may contain not only CD4+/CD8+ immature T cells but also CD4-/CD8- immature T cells, expanding the immunophenotypic spectrum of this entity. © 2019 International Clinical Cytometry Society.