Positron emission tomography (PET) has been found useful in monitoring response to treatment of malignant lymphoma. We investigated the ability of interim PET to monitor response to standard dose R-CHOP chemotherapy in chemotherapy-naïve patients with diffuse large B-cell lymphoma (DLBCL). Between March 2004 and April 2009, 155 DLBCL patients treated with R-CHOP and available for interim and post-treatment PET/CT were identified and included in this analysis. Response, progression-free survival (PFS), and overall survival (OS) were compared between interim PET/CT-negative and positive group, and among three patient groups which were categorized based on their interim and post-treatment PET/CT: those with early metabolic complete response (mCR), delayed mCR, and never mCR. Interim PET/CT-negative patients (n=100) showed superior CR rates to interim PET/CT-positive patients (n=55; 93% vs 62%, P<0.001). However, there was no difference in PFS (P=0.07) and OS (P=0.24) between interim PET/CT-negative and positive group. We categorized patients into three groups, with 100 (64%) in the early mCR group, 35 (23%) in the delayed mCR group, and 20 (13%) in the never mCR group. Early mCR and delayed mCR group did not differ significantly in PFS (P=0.84) or OS (P=0.20). However, the survival outcome in the never mCR group was significantly inferior to the combined early and delayed mCR group. The result from this study suggests that interim PET/CT might be an inappropriate tool for designing risk-adaptive therapy in chemotherapy-naïve DLBCL patients treated with R-CHOP. Prospective trials should be performed to clearly determine the role of interim PET/CT.