Toxic synovitis, an acute inflammatory condition of the hip, is the most common cause of limp and acute pain of the hip in children under 10 years of age. Usually, the synovitis and joint effusion are present unilaterally. The etiology of the condition is unknown, although in a few cases a recent history of an upper respiratory tract infection may be present. The child with toxic synovitis may complain of a limp and pain in the hip, the anteromedial aspect of the thigh, and the knee. The white cell count and erythrocyte sedimentation rate may be slightly elevated, as is the body temperature. Ultrasound is recommended as the primary imaging tool in the diagnosis and treatment of toxic synovitis. Septic arthritis, Perthes disease, and osteomyelitis are a few of the differential diagnoses that the practitioner should consider. Most cases can be managed with bed rest at home and administration of a nonsteroidal anti-inflammatory medication. Follow-up care should occur 2 weeks after diagnosis to ensure there is no recurrence of the joint effusion or progression to avascular necrosis. Radiographs of the hip should be repeated at 1 month and 3 months to complete the patient follow-up.