The goals of this study were to correlate FDG uptake with cell proliferation and cellular density in non-small cell lung cancer. Thirty-one patients with 32 non-small cell lung cancers were examined with FDG PET. For semiquantitative analysis, standardized uptake values (SUVs) were calculated. All patients underwent thoracotomy within 4 wk after the FDG PET study. Cell proliferation was immunohistochemically assessed as the relative number of cells expressing the proliferating cell nuclear antigen ([PCNA] labeling index). Cellular density was also evaluated using light microscopy. SUVs correlated significantly with PCNA labeling index (r = 0.740; P < 0.0001) but only weakly with cellular density (r = 0.392; P = 0.0266). High FDG uptake correlated with high PCNA expression. The PCNA labeling index and SUVs were significantly lower in bronchioloalveolar carcinomas (n = 8) (12.3 +/- 9.45% and 1.45 +/- 0.76, respectively) than in nonbronchioloalveolar carcinomas (n = 19) (33.5 +/- 21.8%, P = 0.015, and 3.75 +/- 1.93, P = 0.003, respectively). However, no significant differences in cellular density were seen between bronchioloalveolar carcinomas and nonbronchioloalveolar carcinomas. FDG uptake is related to cell proliferation rather than to the cellular density of non-small cell lung cancer.