T lymphoblastic leukemia/lymphoma (T-ALL/LBL) is highly aggressive. Although intensive chemotherapies such as ALL-type regimens are commonly used, about half adult patients eventually relapse and die of T-ALL/LBL. Overwhelming evidences have confirmed that interim PET can predict survival outcomes and guide subsequent treatments in Hodgkin lymphoma. However, whether interim PET-CT can predict survival outcomes or not in T-ALL/LBL patients remains unclear. 47 adult patients of T-ALL/LBL were retrospectively reviewed. Interim PET-CT was done after induction therapy and evaluated according to the International Harmonization Project criteria. After induction therapy, interim PET-CT was positive in 19 patients (40.4%). After a median follow up time of 34 months, the 2-year and 3-year progression free survival (PFS) rate were 39% and 30%, respectively, and the 2-year and 3-year overall survival (OS) rate were 54% and 45%, respectively. Using Kaplan-Meier survival analysis, it was found that interim PET-CT positivity correlated with significantly inferior PFS and OS (2-year PFS rate for patients with positive or negative interim PET were 21.1% or 56.0%, respectively, p = 0.002; 2-year OS rate for patients with positive or negative interim PET were 31.6% or 63.7%, respectively, p = 0.010). However, there was no significant relationship between PFS, OS and bone marrow infiltration, lactate dehydrogenase level, and stages (p > 0.05). Interim PET-CT may predict PFS and OS in adult patients of T-ALL/LBL, which needs to be validated in prospective clinical trials. The optimal criteria for interim PET-CT evaluation and riskadapted treatment strategy determined by interim PET-CT should be investigated in future clinical practice.