A patient with Guillain-Barré syndrome was successfully treated by plasma exchange (without any other therapy), and the relationship between changes in his peripheral blood lymphocyte subpopulations and the clinical improvement induced by plasma-pheresis was studied. At the recovery, the most interesting findings were an increase in T suppressor lymphocytes, and in Fc receptor-bearing lymphocytes (above normal values), together with a decrease in B lymphocytes. In this patient, plasma exchange not only removed toxic humoral factors, but also restored a normal balance among peripheral lymphocytes.