In the past decade, rituximab in combination with polychemotherapy has become the standard approach in most patients with advanced CD20-positive B-cell lymphoma. In mantle cell lymphoma (MCL), follicular lymphoma and diffuse large B-cell lymphoma, rituximab has been used as monotherapy and in combination with various chemotherapy regimens in different treatment situations. Routinely, rituximab is given intravenously, but other routes of administration have also been described. Here, we report a 64-yr-old woman who was previously treated with three different chemotherapy regimens for stage IV MCL. No sustained clinical and radiological response could be achieved. The patient's general status declined and she developed massive ascites as the dominant clinical problem. Local, intraperitoneal administration of rituximab was initiated as an experimental treatment approach. After 11 doses of rituximab, the general status of the patient improved significantly, ascites resolved completely and computed tomography (CT) scans demonstrated a partial remission of intra-abdominal lymph nodes and splenomegaly. Furthermore, we observed a regression of mediastinal lymph nodes, pleural effusion and centrocytes in peripheral blood as well as improvement of anaemia. The response to the experimental treatment has maintained for more than 6 months. In summary, we observed a sustained local and systemic response to intraperitoneal administration of rituximab in a patient with heavily pretreated advanced MCL.