A 63 year old woman with non-Hodgkin lymphoma presented with unilateral pleural effusion, which when aspirated revealed CD19 and CD20 positive malignant cells. Prior to this, the patient had received several lines of chemotherapy (CHOP, VAD, FED) with no effect on pleural effusion. Repeated percutaneous drainage procedures were unable to control the effusion either. Rituximab was therefore instilled in a dose escalating manner via repeated pleurocenteses. Fifty days after the application of rituximab, pleural effusion was still present but reduced in size. Flow cytometry and immunocytochemistry performed on the same day showed CD19 positive cells which were lacking CD20 epitope, which could be explained by either engagement or destruction of the CD20 epitope upon interaction with rituximab making the detection of the CD20 molecule impossible by routine flow cytometry. What is especially interesting is the fact that even 50 days after the application of rituximab intrapleurally no new CD20 positive cells could be found in the pleural effusion by immunochemistry or flow cytometry, opening an interesting issue concerning the length of rituximab's activity when applied locally. Although our patient had no adverse effects, further analysis of rituximab's activity and safety when applied intrapleurally is warranted.