The histopathological appearances of the tendon sheath and synovium from 23 patients treated surgically for de Quervain's disease are described and compared with 24 controls. The condition was not characterized by inflammation, but by thickening of the tendon sheath and most notably by the accumulation of mucopolysaccharide, an indicator of myxoid degeneration. These changes are pathognomonic of the condition and are not seen in control tendon sheaths. The term 'stenosing tenovaginitis' is a misnomer and we believe that de Quervain's disease is a result of intrinsic, degenerative mechanisms rather than extrinsic, inflammatory ones.