Lambert-Eaton myasthenic syndrome: An autoimmune disease characterized by weakness and fatigue of the proximal muscles (those near the trunk), particularly the muscles of the pelvic girdle (the pelvis and hips) and the thighs, with relative sparing of eye and respiratory muscles. Lambert-Eaton myasthenic syndrome (LEMS) is associated in 40% of cases with cancer, most often with small cell cancer of the lung and less often with other tumors. The neuromuscular defect in LEMS is due to insufficient release of the neurotransmitter acetylcholine by nerve cells.

LEMS has been treated with pyridostigmine bromide (Mestinon) to increase the transmission of acetylcholine across the neuromuscular junction, a drug called diaminopyridine (DAP) and immunosuppressants (the steroid prednisone, azathioprine, cyclosporine). Plasma exchange provides improvement in some patients with LEMS, as may intravenous immunoglobulin (IVIg). Patients over 50 with a history of long-term smoking are most likely to have an associated tumor. If the tumor is cured, the LEMS may vanish.

LEMS is a "myasthenic syndrome" because the muscle weakness in LEMS is reminiscent of that in myasthenia gravis. Unlike myasthenia gravis, as muscle contractions continue, strength increases in LEMS. The disease is named for Lambert and Eaton who (together with Rooke) described it in 1966. The disease had actually been reported by Anderson and coworkers in 1953 in a man with oat cell cancer of the lung.