Nineteen double-filtration plasma exchanges were performed in 8 patients selected according to the presence or absence of disease-related and/or patient-related risk factors. The haemofiltration system consists of 2 filters with different pore sizes. The first filter, called plasma separator (Asahi plasmaflo HI-05), separates plasma from whole blood; the second filter, called plasma filter (Asahi XK-60; Kuraray Eval 2A or 4A), separates high molecular weight components from plasma. The filtrate from the second filter is returned to the patient mixed with blood cells and either a 4% albumin solution or Plasmion to replace the plasma discarded (about 0.5-0.8 I for a 1-1.5 plasma mass treated). The system (a) modulates venous pressure and transmembrane pressure in each of the two filters by means of pump velocity variations; (b) recirculates and concentrates the plasma extracted, and (c) provides information on the plasma mass extracted by the second filter. In our study, antihistaminics were always infused before each plasma exchange session, and blood pressure and electrocardiogram were monitored throughout the session. The selectivity of the second filter is relatively good for low and high molecular weight components (e.g. albumin and IgM respectively), but needs to be improved for those of intermediate molecular weight, such as IgG and immune complexes. The amounts of plasma substitute utilized are about 6 times less than with conventional methods; however, transmembrane pressure in the second filter is imperfectly controlled, and this too calls for improvement. A study is in progress to evaluate the ideal plasma mass to be extracted. Clinically, and taking into account the biological results obtained, diseases with high molecular weight mediators should benefit from the double-filtration technique, but this technique needs to be perfected for the treatment of IgG-mediated diseases.