To evaluate the occurrence of hemolysis during double-filtration plasmapheresis (DFP) treatment and its causes, we prospectively treated 113 myasthenic patients and monitored transmembrane pressure (TMP), arterial line pressure (Pa), and venous line pressure (Pv) at 30-minute intervals (M30, M60, M90, and M120) during treatment. Laboratory and clinical parameters related to the development of hemolysis were also analyzed. The overall frequency of hemolysis was 35.4%. Peripheral venous access was associated with a higher rate of hemolysis (45.3%) The mean TMP increased significantly after M30 for the hemolysis group compared with the no hemolysis group. A similar trend was present for Pa. Pv did not differ between the 2 groups during treatments until M90 and M120. Pa and TMP changed significantly with time (both P < .0001). Vascular access through large veins and stable TMP during the procedure may help avoid hemolysis.