Toxic synovitis is an irritation and swelling of the lining of the hip joint. The condition causes pain and tenderness that may move down to the thigh and knee; it sometimes makes a person limp.

Toxic synovitis is the most common cause of hip pain in children. It occurs most often in boys between the ages of 4 and 10 years. It commonly follows a mild injury or an illness with low-grade fever, such as tonsillitis or an ear infection.

This condition usually resolves on its own, but treatment may be given for pain and inflammation.

Toxic synovitis is also called transient synovitis.