Five patients with Eaton-Lambert syndrome (ELS) without associated malignancy were first treated by plasmapheresis alone, and subsequently by plasmapheresis combined with prednisone and azathioprine therapy. Three of the five were also treated by the immunosuppressive drugs alone. Although all three therapeutic regimens resulted in some clinical and electromyographic (EMG) improvement, the greatest improvement was seen during plasmapheresis combined with prednisone and azathioprine. The initial evoked compound muscle action potential (CMAP) increased during treatment in all patients, while facilitation of the initial CMAP by tetanic stimulation decreased. Overall, the EMG decrement elicited at 2 Hz lessened in four of the five patients. Serious complications probably attributable to guanidine toxicity developed in three patients: two had renal failure and one had gastrointestinal and urinary tract bleeding. Our results suggest that immunosuppressive therapy may have a place in the management of ELS and that circulating factors such as autoantibody may participate in its pathogenesis.