Of 59 patients (2-15 years of age) with acute synovitis of the hip, one patient developed Perthes disease (1.7%) and the others transient synovitis. The diagnostic criterion for intracapsular effusion was a side difference greater than or equal to 2.0 mm in the distance between the anterior joint capsule and the femoral neck (anterior capsule distance, ACD). At the time of admission, the mean ACD of the affected and unaffected hips was 8.2 and 4.2 mm, respectively. The effusion persisted for greater than 1 week in 58% of the patients. We recommend ultrasound as the main imaging technique in the diagnosis and follow-up of transient synovitis. Radiography may be omitted in uncomplicated cases, but should be employed when Perthes disease is suspected.