A childhood disorder that causes inflammation in the hip joint. Toxic synovitis is most common between the ages of 3 and 6 and more frequently affects boys than girls. Although its exact cause remains unknown, toxic synovitis is most likely connected to a viral infection. A limp and hip pain (usually on one side of the body) are the most common symptoms of toxic synovitis. Often an affected child has a viral illness, such as a cold or flu. There may also be a low-grade fever of up to 101°F and associated pain in the knee or thigh of the affected leg. Symptoms usually disappear within 1 to 4 weeks.