[18F]fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) is useful for primary staging and evaluation of treatment outcome in diffuse large B cell lymphoma (DLBCL) patients. The reduction in the maximum standardized uptake value (ΔSUVmax) from the initial to the interim 18F-FDG PET scan has been reported to predict survival in DLBCL patients. We retrospectively evaluated ΔSUVmax obtained by PET or PET-computed tomography before and after initial therapy in 31 newly diagnosed DLBCL patients who were treated with R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone) therapy. Receiver observation characteristic curve analysis was used to evaluate the optimal cutoff value for the ΔSUVmax for disease progression. The 3-year progression-free survival rate of patients with ΔSUVmax≥83 and <83% was found to be 91 and 25%, respectively (P<0.001). The 4-year overall survival rate of patients with ΔSUVmax≥83 and <83% was found to be 100 and 83%, respectively (P=0.046). The ΔSUVmax observed before and after R-CHOP therapy could be useful in the prediction of disease progression and survival in newly diagnosed DLBCL patients.