Aims: It was the aim of this paper to identify prognostic factors in patients with relapsed or refractory B-cell non-Hodgkin's lymphomas, treated by radioimmunotherapy (RIT) with radioiodinated human/murine chimeric anti-CD20 monoclonal antibody rituximab (131I-rituximab). Methods: Twenty-four patients were enrolled prospectively and were treated with unlabeled rituximab 70 mg and a therapeutic activity (median 7.3 GBq) of 131I-rituximab. Contrast-enhanced 18F-FDG PET/CT scans were performed before and after 1 month of RIT. Tumor sizes and maximum standardized uptake values (SUVmax) of scans were measured. Results: Four of the 24 patients survived. High SUVmax in a pretreatment scan was found to be related to poorer overall survival (OS) and progression-free survival (p = 0.04 and 0.02, respectively). Furthermore, a large tumor size in a pretreatment scan was associated with poorer OS but not with progression-free survival (p < 0.01 and p = 0.07, respectively). By multivariate analyses, a high SUVmax, a large tumor size in a pretreatment scan and diffuse large B-cell lymphoma histology were significantly associated with poorer OS [p = 0.04/hazard ratio (HR) = 3.54, p < 0.01/HR = 5.52, and p = 0.02/HR = 3.38, respectively). Conclusion: SUVmax and tumor size determined by a pretreatment 18F-FDG PET/CT result as significant predictors of OS in patients with relapsed or refractory B-cell non-Hodgkin's lymphoma treated by RIT.