Standardized uptake value (SUV) is a marker of tumor glucose metabolism detected by [18F]-fluorodeoxyglucose positron emission tomography/computed tomography ((18)F-FDG-PET/CT). The maximum SUV of the whole-body (BmSUV(max)) reflects the tumor aggressiveness in non-Hodgkin lymphoma. To evaluate the correlation between SUV(max) at biopsy site (BxSUV(max)) and proliferation potential of tumor cells in untreated diffuse large B-cell lymphoma (DLBCL), fifteen pre-therapy PET/CT scans in patients with histologically proven DLBCL were retrospectively analyzed together with Ki-67 proliferation index. The BmSUV(max) and BxSUV(max) were evaluated from the fused PET/CT images. Ki-67 proliferation index was measured in the biopsy specimens using an immunohistochemical technique in archival paraffin-embedded sections. The BmSUV(max) was significantly higher than the BxSUV(max) (mean 19.6 vs.16.6, p < 0.01). The BxSUV(max) correlated with the Ki-67 proliferation index (r = 0.7, p < 0.01), but no correlation was detected between the BmSUV(max) and the Ki-67 proliferation index. The results indicate that tumor proliferation potential might be predicted in vivo by FDG-PET/CT images. Thus, PET/CT is useful to guide biopsy by selecting sites with the BmSUV(max) when clinically appropriate.