Transient synovitis of the hip (a.k.a. Toxic synovitis,Transitory coxitis, Coxitis fugax, Acute transient epiphysitis, Coxitis serosa seu simplex, Phantom hip disease, Observation hip and Irritable hip) is a condition which causes hip pain due to inflammation of the inner lining of the hip joint.
It was first described in 1892, and is a benign, self-limiting condition which usually resolves in a week or so, with or without treatment with NSAIDs (e.g.)
It usually affects children between the ages of 3 and 10, (around 2 - 3 % ) leading to hip pain and limping. It has been noted that it sometimes follows upper respiratory tract infections - viral and bacterial.
What leads to the inflammation, and why it appears to be restricted to young children, is unknown.
More info : Wikipedia