Lambert-Eaton myasthenic syndrome (LEMS), an autoimmune disorder that affects the neuromuscular junction, is characterized by proximal muscle weakness, reduction of tendon reflexes, and autonomic dysfunction. LEMS shows a prevalence of approximately 0.25-0.27 per 100,000 population. The characteristic muscle weakness observed in patients with LEMS is attributed to the role of pathogenic autoantibodies directed against voltage-gated calcium channels (VGCC) present on the presynaptic nerve terminal. Notably, 50-60% of patients with LEMS have an associated tumor, small-cell lung carcinoma (SCLC), which also expresses functional voltage-gated calcium channels (VGCC). The Japanese LEMS diagnostic criteria 2022 recommend documentation of typical electrophysiological abnormalities combined with myasthenic symptoms for accurate diagnosis. P/Q-type VGCC antibody positivity strongly supports the diagnosis. Treatment options are categorized as oncological treatment, immunotherapy, and symptomatic treatments. Effective treatment of the tumor can improve LEMS in patients with SCLC. Most patients benefit from 3,4-diaminopyridine administration for symptomatic treatment. A treatment algorithm is established by the clinical practice guidelines 2022.