BACKGROUND: Maximum standardized uptake value (SUVmax) and maximum tumor diameter (MTD) have been shown to reflect survival outcome in diffuse large B cell lymphoma (DLBCL). However, applying these values to primary extranodal DLBCL is difficult because they are separate nosological entities with differences in genetic origin. We therefore decided to evaluate whether SUVmax and MTD on 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (18-FDG) positron emission tomography (PET) would affect the survival outcome in primary extranodal DLBCL. METHODS: From October 2005 to November 2010, 76 primary extranodal DLBCL patients receiving R-CHOP therapy were analyzed. All patients had undergone an initial 18-FDG PET/CT and conventional computed tomography (CT) of the neck, chest, abdomen, and pelvis for staging. Median follow-up period was 35 months. RESULTS: The SUVmax and MTD cut-off values were 11.0 and 7.5 cm, respectively. SUVmax> or =11.0 predicted a short progression free survival (PFS, P=0.002) and overall survival (OS, P=0.002). MTD> or =7.5 cm was associated with poor PFS (P=0.003) and OS (P=0.003). High International Prognostic Index (IPI) was also associated with the survival outcome (PFS, P=0.046; OS, P=0.030). Multivariate analysis revealed that SUVmax> or =11.0 (PFS, hazard ratio [HR]=10.813, P=0.024; OS, HR=6.312, P=0.015); MTD> or =7.5 cm (PFS, HR=5.631, P=0.008; OS, HR=4.072, P=0.008); and high IPI (PFS, P=0.027; OS, P=0.046) were independent prognostic factors. CONCLUSION: It appears that both MTD and SUVmax can be independent prognostic factors in primary extranodal DLBCL.