LEMS is a presynaptic neuromuscular junction disorder typically associated with small cell lung carcinoma. The characteristic electrophysiological abnormality is a low amplitude compound muscle action potential that shows a marked increment after short maximal contraction or brief tetanic nerve stimulation. Here we describe 3 patients who had LEMS in association with lymphoproliferative disorder. The first patient had Castleman's syndrome with typical clinical and electrophysiological features of LEMS, which responded partially to treatment with 3-4-diaminopyridine. The second patient was a 7-year-old boy who had an unusual acute onset of LEMS associated with relapse of his Burkitt's leukemia. The third patient was a 60-year-old woman with non-Hodgkin's lymphoma. These 3 patients (together with 6 additional patients identified in the literature) lead us to suggest that lymphoproliferative diseases are another, hitherto unrecognized, type of malignancy associated with LEMS. Thus, any patient with these malignancies and unexplained muscle weakness should have electrophysiological evaluation for LEMS.