The side effects of a series of 2418 hemapheresis procedures performed in a total of 570 subjects (patients and donors) are described. Patients with various diseases were subjected to plasmapheresis (926 procedures in 181 patients) or cytapheresis (305 procedures in 89 patients). One hundred twelve plasmapheresis procedures and 1075 of cytaphereses were also performed in 300 blood donors. A total of 225 complications involving 107 patients (39.6%) occurred during 196 (15.9%) therapeutic procedures. Among the blood donors only 45 complications, involving 35 patients (11.6%) occurred during 45 procedures (4.2%). The complications seen with therapeutic plasmaphereses were circulatory disturbances (38% of all those observed), citrate reactions (27%), technical problems (20%), allergic reactions (9%) and miscellaneous complications (6%). Therapeutic cytaphereses were complicated by citrate reactions (44%), technical problems (25%), circulatory disturbances (14%), allergic reactions (11%) and miscellaneous complications (6%). Complications in the blood donor group included circulatory disturbances (51%), technical problems (36%) and various other problems (13%). No infectious complications or deaths were observed. The probability that adverse reactions will occur depends on the condition of the patient, the frequency of the sessions and the volume of fluid exchanged. Evaluation of the main risk factors, use of less intensive protocols and interruption of the session at the first sign of disturbances will help improve patient tolerance of these procedures.