Background: Syncytin-1 is improperly expressed in several cancers. However, its expression profile across leukocytes in leukemia patients has not yet been analyzed. Methods: A total of 50 AML cases and 14 B-cell ALL patients were consecutively recruited. Bone marrow samples were subjected to flow cytometry. Statistical analysis was applied to compare syncytin-1 expression between B-cell ALL and AML across granulocytes, leukemia cells, and T-lymphocytes (including CD3+, CD4+, and CD8+ subsets thereof) and to correlate syncytin-1 expression to leukemia cells and lymphocytes with the T-cell subset percentages. Results: The syncytin-1-expressing leukemia cell% in AML patients was significantly higher than that in B-cell ALL patients (p < 0.05). The CD8+ T-cell% in AML patients was significantly higher than that in B-cell ALL patients (p < 0.05). The syncytin-1 expression rate on leukemia cells was positively correlated with the CD8+ T-cell percentage (r = 0.289, p < 0.05), while the syncytin-1 expression rate on lymphocytes was negatively correlated with the CD3+, CD4+, and CD8+ T-cell percentages (r = -0.273, -0.450, and -0.307, respectively; p < 0.05). Conclusions: The percentage of syncytin-1-expressing leukemia cells in AML - due to its positive correlation with the CD8+ suppressor T-cell percentage -shows potential as an indicator of poorer long-term immunity in AML patients.