This two-arm, randomised, multicentre, open-label, phase IIIb study investigated the safety and efficacy of a 3-h catumaxomab infusion with/without prednisolone premedication to reduce catumaxomab-related adverse events. Patients with malignant ascites due to epithelial cancer received four 3-h intraperitoneal catumaxomab infusions with/without intravenous prednisolone (25 mg) premedication before each infusion. The primary safety endpoint was a composite safety score calculated from the incidence and intensity of the most frequent catumaxomab-related adverse events (pyrexia, nausea, vomiting and abdominal pain). Puncture-free survival (PuFS) was a co-primary endpoint. Time to next puncture (TTPu) and overall survival (OS) were secondary endpoints. Prednisolone premedication did not result in a significant reduction in the main catumaxomab-related adverse events. The mean composite safety score was comparable in both arms (catumaxomab plus prednisolone, 4.1; catumaxomab, 3.8; p = 0.383). Median PuFS (30 vs. 37 days) and TTPu (78 vs. 102 days) were shorter in the catumaxomab plus prednisolone arm than in the catumaxomab arm, but the differences were not statistically significant (p = 0.402 and 0.599, respectively). Median OS was longer in the catumaxomab plus prednisolone arm than in the catumaxomab arm (124 vs. 86 days), but the difference was not statistically significant (p = 0.186). The superiority of catumaxomab plus prednisolone versus catumaxomab alone could not be proven for the primary endpoint. Prednisolone did not result in a significant reduction in the main catumaxomab-related adverse events. The study confirms the safety and efficacy of catumaxomab administered as four 3-h intraperitoneal infusions for the treatment of malignant ascites.