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Clinical and biological parameters were retrospectively reviewed in 34 cases of T-lineage acute lymphoblastic leukemia (T-ALL), classified as ““early”” (20 cases) or ““late”” (14 cases) subgroups, according to the degree of blast cell differentiation, assessed by immunophenotyping.In ““early”” T-ALL, age, co-expression of ““immature”” (CD34 and HLA-Dr) or myeloid (My+) anti-gens, proliferative activity (as evaluated by Ki67 monoclonal antibody), and expression of the ““multidrug-resistance”” (MDR) phenotype (as determined by C-219 monoclonal antibody) were significantly higher, while WBC count and expression of CD10 were significantly lower, than in ““late”” T-ALL. Furthermore, although no statistically significant difference was found between the two groups, ““late”” T-ALL more frequently displayed a greater extramedullary tumor mass (““lymphoma-like”” syndrome), LI FAB morphology and a normal karyotype. A single patient, with ““late”” T-ALL, also showed positivity for TCR gamma/delta chains, specific monoclonal antibodies.On the whole, 30 patients (88.2%) achieved complete remission: 16 (80%) were ““early”” and 14(100%) ““late”” T-ALL. No statistical difference was found between the two groups with respect to disease free survival (42% vs 54% at six years), whereas median overall survival was significantly shorter in ““early”” T-ALL (23 months vs median not yet reached at six years for ““late”” T-ALL, p < 0.05). We conclude that ““early”” and ““late”” T-ALL show clinical and biological differences, that could perhaps justify differential therapeutic approaches.