Anterior interosseous nerve (AIN) injuries account for only 1% of all the nerve injuries at the upper limb. We report the case of a 22-year-old male who sustained a penetrating injury to the arm. No neurological deficit was found at the initial evaluation. However, 6weeks later, he had a motor deficit confined to the territory of the AIN with weakness of the flexor pollicis longus and flexor digitorum longus to the index. He also reported paraesthesia. Tinel's test was positive over the pinpoint wound in the arm, where a painful swelling was felt. Electroneurophysiological testing indicated a deficit of the AIN. Surgical exploration identified a thrombosed false aneurysm of the humeral artery responsible for compression of the median nerve. One month later, the patient had achieved a full recovery. Immediate routine exploration of deep penetrating wounds, although mandatory, may fail to detect any lesions. Close monitoring must be provided subsequently, as gradual nerve compression can result in delayed neurological deficits.