In the past, treatment for malignant ascites focussed on symptomatic relief or treatment of the underlying disease. A promising option evolved with catumaxomab (Removab^®) in 2009. Since catumaxomab is a bispecific, trifunctional antibody with mouse-rat origin, so far repeated treatment cycles were not an option due to the occurrence of human anti-drug antibodies (HADA). Nevertheless, the good results obtained so far raised the question whether a repeated treatment cycle with catumaxomab could be feasible and effective. We report on a 74-year-old female patient with breast cancer and peritoneal carcinomatosis. To our knowledge, this is the first patient world-wide to be treated with a repeated cycle of catumaxomab. HAMA (human anti-mouse antibodies) values were identified in blood and ascites samples. Ascites samples were also stained to identify and quantify cells, positive for EpCAM (epithelial cell adhesion molecule) and CD45. The patient tolerated the second cycle without unexpected side effects and remained puncture-free for another 45 days. Analysis of blood and ascites revealed a quick increase in HAMA values in the blood samples after 1 week, but considerably lower HAMA values and delayed increase in the ascites samples. Also a distinct and continuous decrease of EpCAM-positive cells was observed in the ascites samples under treatment. A strong increase in CD45-positive cells was detected after the beginning of the second cycle, with a consecutive decline toward the end. This first experience suggests that a repeated cycle of catumaxomab might be feasible and effective. As a consequence, a phase II trial (SECIMAS) was initiated.