A 76-year-old woman developed right carpal tunnel syndrome after being conservatively treated for tenosynovitis of the flexor tendons with associated mild carpal tunnel syndrome. A magnetic resonance imaging scan showed a tumour in the carpal tunnel. Re-exploration showed that the median nerve was being compressed by a giant cell tumour of the flexor tendon sheaths. Appropriate imaging is advised in patients with additional findings (such as swelling) or in patients with secondary carpal tunnel syndrome and incomplete response to conservative treatment, to exclude a space-occupying lesion.