The maximum standard uptake value (SUVmax) of the whole body on 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) reflects the tumor aggressiveness in non-Hodgkin lymphoma (NHL). To clarify the correlation between SUVmax at the biopsy site and the proliferation potential of tumor cells, we studied 36 patients with untreated NHL and five with untreated Hodgkin lymphoma (HL) by measuring the Ki-67 proliferation index (MIB-1 labeling index) in biopsy specimens. The measured MIB-1 labeling index was categorized into seven levels: nearly 0%, 5-20%, 21-40%, 41-60%, 61-80%, 81-95%, and nearly 100%. Twenty-four lymph nodes (LNs) and 17 extranodal (EN) sites were biopsied. The reviewed diagnosis was eight indolent lymphomas, two mantle-cell lymphomas, 26 aggressive lymphomas, and five HLs. A positive correlation was observed between the SUVmax at the biopsy site and the MIB-1 labeling index in the 36 patients with NHL (r = 0.69, p < 0.001). The correlations were also observed in LN group (r = 0.60, p = 0.006) and EN group (r = 0.87, p < 0.001), respectively. In the five patients with HL, the MIB-1 labeling index was uniformly categorized in nearly 100%. The SUVmax correlates with the proliferation potential in the case of NHL.