A 17-year-old man with no familial history developed motor and sensory disturbance of the left upper limb a few days after starting push-up exercise. Neurological examination revealed broad weakness and radial sensory disturbance of the left upper limb and magnetic resonance neurography showed laterality of brachial plexus intensity signals. Therefore, we suspected left brachial plexopathy. However, a nerve conduction study showed a broad disturbance that could not be explained by only brachial plexopathy. Genetic tests revealed a diagnosis of hereditary neuropathy with liability to pressure palsies (HNPP). HNPP should be included in the differential diagnosis for neuropathy due to slight exercise or nerve compression even when familial history is negative.