Rationale:Approximately 0.001% of patients with cancer have paraneoplastic nerve system syndrome, which can affect the central nervous system, neuromuscular junction, or peripheral nervous system. Although myasthenia gravis (MG) may exist as a thymic paraneoplastic syndrome (PNPS), its association with primary lung cancer remains unknown.Patient concerns:A 55-year-old female presented with slurred speech, weakness in chewing, sporadic difficulty in swallowing, and weakness in both lower limbs for half a year.Diagnoses:Based on cerebrospinal fluid and electromyography findings, we present the case of a female patient diagnosed with overlapping multicranial nerve tumor infiltration and MG-like neurological PNPS secondary to lung adenocarcinoma.Interventions:The patient received intrathecal injections of pemetrexed and neurotrophic (vitamin B) therapy before ceasing chemoradiotherapy and chose cabozantinib on her own.Outcomes:Weakness of the proximal limbs, choking cough, and chewing problems did not improve significantly.Lessons:Although it is unclear why MG coexists with lung cancer, it is probable that MG is a paraneoplastic condition. Cerebrospinal fluid testing should be carried out along with electrophysiological, serological, and pharmacological procedures pertinent to the diagnosis of MG to thoroughly examine if people simultaneously experience MG-like PNPS and tumor growth. Starting immunotherapy and anticancer medication at the same time that tumor development and MG-like syndrome are discovered is crucial.