Guillian-Barré syndrome (GBS) is an immune mediated disease which is most commonly caused by infections. Symptoms are rapidly progressive and may include servere weakness of the extremities, bulbar weakness, autonomic dysfunction and respiratory insufficiens. In rare cases paraneoplastic syndrome may mimic GBS, which is important to know as treatment will fail unless the underlying malignancy is treated. This is a case report of paraneoplastic non-small cell lungcancer mimicking GBS with effect of the checkpoint inhibitor pembrolizumab.