Although a few studies have evaluated the utility of ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) in treatment-naive T-cell non-Hodgkin's lymphomas (T-NHLs), a few studies had been conducted to evaluate the role of FDG-PET in patients after treatment. A total of 41 patients with T-NHLs were included who underwent post-autologous stem cell transplantation (ASCT) PET for response assessment in addition to contrast-enhanced CT. The impact of post-ASCT PET on response assessment and predicting prognosis was retrospectively evaluated. Of the 41 patients, 11 (26.8%) patients showed discordant response between two image modalities (Cohen's κ = 0.465). The additional PET study actually guides changing the post-ASCT response in six (54.5%) of these 11 patients. Moreover, positive post-ASCT PET was associated with worse prognosis in event-free survival and overall survival than negative post-ASCT PET (P = 0.003 and P = 0.044, respectively). Excluding four patients in whom further confirmation was not available due to early mortality, in 22 lesions showing discrepancy between two image modalities, 12 lesions were true positive (N = 4) or true negative (N = 8) for PET and ten lesions were false positive (N = 7) or false negative (N = 3). The accuracy for the discrepant lesions was 54.5% (12 of 22), the overall accuracy for the detected lesions was 76.7% (33 of 43), and the overall accuracy for patients was 89.2% (33 of 37). Post-ASCT PET was useful in response assessment after ASCT, and the positive post-ASCT PET result was associated with worse prognosis than the negative post-ASCT PET in patients with T-NHLs.