Maintenance hemodialysis is widely used throughout the world, and anaphylactic reactions appear to be increasing in number and severity. However, the exact incidence of sensitization and the role of atopy in these reactions are not yet fully understood. All of the 111 patients routinely dialyzed in a center were tested. All patients had a complete investigation of atopy, RAST to chemicals released during the procedure of dialysis (ethylene oxide (Eto), formaldehyde, phthalic anhydride, and toluene diisocyanate), skin tests with the effluent, and the titration of blood eosinophils. The incidence of atopy was found to be lower (13.5%) than in the normal population of the area. Skin tests with either histamine or allergens are significantly (p less than 0.001) smaller than those of nondialyzed subjects, and this method does not appear to be ideal in this population of patients. Eto sensitivity ranked first (5.5%), followed by phthalic anhydride sensitivity (3.6%); 5/6 patients who had a sensitivity to Eto and/or phthalic anhydride presented symptoms during dialysis, but they never were life threatening. Formaldehyde RAST was only found in one patient who had a life-threatening reaction. Finally, three patients presenting pruritus had positive skin prick tests with the effluent of the dialyzer. All patients having a first use syndrome and 80/81 symptom-free patients did not have serum-specific IgE against the released chemicals, 5/17 patients who had a pruritus during dialysis had either positive RAST to released chemicals or skin tests to the effluent, 5/8 patients who suffered from anaphylaxis had positive RAST to released chemicals, but only those who had a positive RAST presented a severe reaction.(ABSTRACT TRUNCATED AT 250 WORDS)