The purpose of this study was to investigate the immunophenotyping characteristics of adult acute lymphoblastic leukemia (ALL) patients in groups of different ages. Immunophenotyping was performed in 260 ALL patients by flow cytometry using a panel of monoclonal antibodies and CD45/SSC gating. The results indicated that (1) all the 82 cases of T-cell acute lymphoblastic leukemia (T-ALL) expressed CD7 (100%) while the positive rate of CD2 remarkably decreased with aging. The positive rate of CD2 in patients aged 14 to 18 years (adolescents) was 91.67%, which is significantly higher than that in cases aged 19 to 35 years (young adults) and > 35 years (older adults) (65.71% and 43.48% respectively, p < 0.05); the positive rate of CD34 and HLA-DR increased with aging, there was significant difference of the HLA-DR expression between the older adults group (39.13%) and the other two groups (4.17% in adolescents and 11.43% in young adults respectively (p < 0.05). Moreover, there were significant differences of the myeloid antigen (MyAg) and CD13 expression between the older adults and younger adults (p < 0.05). (2) As to adult B-cell acute lymphoblastic leukemia (B-ALL), the positive rates of CD19 and HLA-DR in 178 cases were 100%; the positive rate of CD33 in young adults was significant higher than that in adolescents (p < 0.05), the differences of the other marker expressions failed to reach statistical significance in adult B-ALL patients. It is concluded that the immunophenotypes of adult T-ALL are evidently heterogeneous in different ages, and expression with more aberrant phenotypes indicates poor prognostic significance in patients older than 35 years. There is no significant association of immunophenotypes with ages among different age groups of adult B-ALL.