We recorded side effects and other complications of 813 plasma exchanges used in early treatment of polyarteritis nodosa and Churg-Strauss angiitis in a prospective study of 72 patients (22-75 years old). All the patients were also treated with a corticosteroid (1 mg/kg/day), and half were included in a randomized trial of cyclophosphamide (2 mg/kg/day during 1 year). Centrifugation was used in 678 plasma exchange sessions (83.4%) and filtration in 128 (15.7%) (no data were available about the technique used in seven cases). The replacement fluid in 745 sessions was 4% albumin and in 115 was fresh-frozen plasma; eight patients received both (47 sessions). Two hundred and fifty-one complications were reported in 60 patients during 206 (25.3%) of the 813 completed exchanges; 47 sessions (5.8%) were temporarily stopped as a result of complications. The most common problems were technical difficulties (in 90 sessions), moderate or severe hypotension (in 52), and allergy to the replacement fluid (in 51). Hepatitis B antigen appeared in one patient. In four patients, plasma exchange was stopped permanently because of the severe side effects. No patient died during a session. Twelve of the 72 patients died during the study, six in each of the two groups. In the group treated by a combination of corticosteroid and plasma exchange, deaths were related to the deleterious effects of the disease itself and occurred after 12.8 +/- 11.1 months (1-26 months). In the group treated by the same combination plus cyclophosphamide, four of the six deaths were due to severe infections, which were related to leukopenia in three patients.(ABSTRACT TRUNCATED AT 250 WORDS)