De Quervain's tenosynovitis is inflammation of tendons on the side of the wrist at the base of the thumb. These tendons include the extensor pollicis brevis and the abductor pollicis longus tendons.

De Quervain's tenosynovitis can be brought on by simple strain injury to the extensor pollicus longus tendon. Typical causes include stresses such as lifting young children into car seats, lifting heavy grocery bags by the loops, and lifting gardening pots up and into place.

De Quervain's tenosynovitis is diagnosed based on the typical appearance, location of pain, and tenderness of the affected wrist. De Quervain's tenosynovitis is usually associated with pain when the thumb is folded across the palm and the fingers are flexed over the thumb as the hand is pulled away from the involved wrist area. (This is referred to as the Finkelstein maneuver.)