OBJECTIVE: To study the clinicopathologic features, immunohistochemical findings and prognosis of precursor lymphoblastic lymphoma/acute lymphoblastic leukemia (LBL/ALL). METHODS: One hundred and fifty-three cases of LBL/ALL were retrospectively analyzed. Immunohistochemical study was carried out. The pathologic findings were correlated with Ann Arbor tumor stage, Ki-67 index, other clinical parameters (including mediastinum/bone marrow involvement, hepato-splenomegaly, age and gender of the patients) and the survival data. RESULTS: Staining for TdT and CD99 was positive in 79.1% (121/153 cases) and 96.3% (131/136 cases), respectively. The cases were categorized into three groups according to the immunohistochemical findings, as follows: precursor T-cell, precursor B-cell and undefined. T-LBL/ALL accounted for 69.3% (106/153 cases) of all of the cases. The male-to-female ratio was 2.4:1 (including 75 males and 31 females). The median age at diagnosis was 17.5 years (ranged from 2 years to 68 years). Ninety-two patients (86.8%) presented with peripheral lymphadenopathy and 59 of them (55.7%) had mediastinal masses. Ninety-one cases (85.8%) were in stage III or IV at diagnosis. The 1-year and 5-year survival rates in patients with T-LBL/ALL were 36.1% and 8.1%, respectively. Patients older than 25 years and those presented in stage III or IV suggested a poor prognosis (P = 0.049 and 0.001, respectively). On the other hand, 29 of the 153 cases (19.0%) belonged to B-LBL/ALL. The median age of the patients was 14 years (ranged from 9 months to 75 years). The male-to-female ratio was 1.6:1 (including 18 males and 11 females). Seventeen patients (58.6%) presented with peripheral lymphadenopathy and 13 of them (44.8%) had involvement of bone marrow or peripheral blood. Mediastinal involvement was found only in 5 cases (17.2%). Twenty-one patients (72.4%) were in stage III or IV at diagnosis. The 1-year and 5-year survival rates were 53.3% and 36.7%, respectively. The remaining 11.7% cases (18/153 cases) were categorized as undefined type, with a negative staining for the following immuno-markers including: CD3ε/CD3, CD45RO, CD79a, CD20, MPO, CD5, CD56, cyclin D1, cytokeratin, neuron-specific enolase, chromogranin A and synaptophysin. The median age of the patients was 15.5 years (ranged from 4 to 53 years). The male-to-female ratio was 2.6:1 (including 13 males and 5 females). The percentage of T-LBL/ALL patients with mediastinal masses were significantly higher than that of B-LBL/ALL cases (P = 0.0003). There was no significant difference in prognostic parameters of T-LBL/ALL and B-LBL/ALL (P = 0.07). The difference in median survival time however was statistically significant (6.0 months +/- 1.1 months versus 15.0 months +/- 7.0 months). CONCLUSIONS: Both TdT and CD99 are useful markers for the diagnosis of precursor lymphoblastic malignancy. T-LBL/ALL predominantly affects children or adolescent males and frequently presents with lymphadenopathy and mediastinal masses, whereas B-LBL/ALL are often accompanied by bone marrow and peripheral blood involvement. In general, T-LBL/ALL carries a poor prognosis. The prognostic criteria include age of older than 25 years and a classification of stage III or IV disease.