A 46 year old man presented with a growing mass over his wrist. Erosions of the triquetrum and hamate were present radiographically. Magnetic resonance imaging (MRI) showed a solid mass arising from the extensor carpi ulnaris tendon, which was T(1) hypointense and isointense, T(2) hypointense, and bloomed on gradient echo images. The preoperative diagnosis of giant cell tumour of the tendon sheath was confirmed on histopathological examination of the excised specimen. The clinical, pathological, and imaging features, with emphasis on MRI findings, of this condition are reviewed.