Purpose: To report the pharmacokinetics of 131I-rituximab a patient with a CD20 positive non-Hodgkin Lymphoma who has received 131I-rituximab as consolidation treatment after remission induction and to evaluate the effect of radioiodination on the biological properties of rituximab. Results: The patient was a 65-year-old male with a relapsed CD20 positive follicular non-Hodgkin Lymphoma. After induction therapy the patient was in partial remission. Following administration of a diagnostic dose of 185 MBq 131I-rituximab, remaining lesions were identified on the wholebody scans. The patient then received a therapeutic dose of 1000 MBq 131I-rituximab. The uptake by the tumor in the right axilla was 0.17-0.21% of the injected dose. The calculated biological half-life of 131I-rituximab was 684 hrs. This biological half-life corresponded well with the half-life of unlabeled rituximab which was approximately 720 hrs. Discussion and conclusion: Even though radioiodination of rituximab results in a reduced binding capacity, whole body scans demonstrated localization of 131I-rituximab in the tumor area. This observation supports the specific targeting of 131I-rituximab. The half-life of 131I-rituximab corresponded to the half-life of unlabeled rituximab. Hence, the pharmacokinetics of 131I-rituximab was not relevantly affected by the radioiodination process.