Non-paraneoplastic Lambert-Eaton myasthenic syndrome (LEMS) is an antibody-mediated autoimmune disorder, in which genetically determined interleukin-10 (Il-10) and tumor necrosis factor-alpha (TNF-alpha) could play a role in the susceptibility for the disease. Therefore, we analyzed the production of Il-10 and TNF-alpha after whole-blood stimulation in first-degree family members of patients with LEMS without malignancy, as a measure of innate production in the patients. Thirty-six first-degree family members of 10 patients and 80 healthy controls were studied. Both Il-10 (p=0.037) and TNF-alpha (p=0.0016) production were increased in the family members, but had no relation with the severity of LEMS or HLA-B8DR3 carriership. Our findings suggest that high innate production of Il-10 and TNF-alpha is a susceptibility factor for non-paraneoplastic LEMS.