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 typically occurs in children between the ages of 3 and 8 years. Toxic synovitis happens more often in boys than in girls. It commonly follows a mild injury or an illness with a 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.