Metastatic infiltration of a peripheral plexus, also named metastatic plexopathy (MP), often results in severe pain and muscular weakness. This rather rare event may have a dramatic impact on the quality of life of patients affected by cancer. We hereby report a rare case of painful MP of the left cervicobrachial plexus presenting as the inaugural manifestation of poorly differentiated large-cell lung carcinoma in a 53-year-old patient. This responsible lung carcinoma was fortuitously diagnosed during MRI of the brachial plexus (BP). Complementary cancer staging was completed by contrast-enhanced multidetector CT, 18-fludeoxyglucose-positron emission tomography/CT and colour Doppler ultrasound of the BP. Although MRI remains the gold standard method for imaging the BP, our reported case emphasizes the alternative diagnostic capabilities of contrast-enhanced multidetector CT and ultrasound and confirms the high specificity of 18-fludeoxyglucose-positron emission tomography/CT in distinguishing brachial MP from secondary radiation plexopathy.