Precursor T-cell lymphoblastic leukemia/lymphoma (T-ALL/LBL) presents a mediastinal mass in one half of cases. Although the immunophenotypic features of T-ALL/LBL have been analyzed in several studies, few studies have been focused on the relationship between the anatomic distribution of lesions and immunophenotypic findings. We analyzed the clinicopathologic findings for 17 patients with T-ALL/LBL diagnosed since 1993 and whose radiologic findings were available. Data on 14 men and 3 women with a median age of 26 years (range, 10-61 years) were analyzed. On the basis of radiologic findings, the cases were divided into thymic type (n = 8) and nonthymic type (n = 9). Patients with the thymic type of T-ALL/LBL had a large mediastinal mass and minimal systemic lymphadenopathy only in the supradiaphragmatic region. Those with the non-thymic type had predominantly systemic lymphadenopathy that included infradiaphragmatic lesions. Expression of CD8 (6/7 versus 0/9) was more frequently found in the thymic type (P < .001), whereas expression of CD56 (0/7 versus 5/9) was more frequent in the nonthymic type (P = .034). In conclusion, T-ALL/LBL was divided into 2 entities, thymic type and nonthymic type, on the basis of radiologic findings and immunophenotypic features. Analysis of the expression of CD8 and CD56 would be useful for biologically classifying T-ALL/LBL into the 2 types. This study was performed in a single institution, was retrospective, and had a limited number of patients; multicenter confirmatory studies are warranted.