Four patients with refractory or poorly responsive chronic progressive demyelinating polyneuropathy (CPDP) were treated with total lymphoid irradiation (total dose, 2000 rad) in an uncontrolled feasibility study. All patients had previously failed conventional therapy for CPDP, as well as other unconventional treatments. During a follow-up period of 7 to 12 months after total lymphoid irradiation, there was a profound and sustained suppression of the absolute lymphocyte count and in vitro lymphocyte function, as well as an increase in the ratio of Leu-2 (suppressor/cytotoxic subset) to Leu-3 (helper/inducer subset) T cells in the blood. Three of the four patients demonstrated improvement in distal muscle strength, and this was associated with increased functional capabilities in two patients. In contrast, no clinical improvement in sensation was noted in any patient. Nerve conduction studies showed patchy improvement in three patients. The results of this preliminary uncontrolled study indicate that radiotherapy deserves further study in the treatment of CPDP.