Transient synovitis of the hip in children is a non-specific inflammatory and self-limited condition. It is also the most common cause of painful hip in children under ten years of age. Despite of the benign prognosis, there are many difficulties in distinguishing it from other diseases of the hip joint. It remains a common diagnostic problem for clinician because the clinical symptoms, physical findings, and conventional radiography is not pathognomonic of the condition. The authors paid attention to the increase of effusion in the affected hip and studied the value of the hip ultrasonography in 24 cases of transient synovitis from August 1985 to July 1987. The results are summerized as follows : 1. The ratio of male to female was 7 to 1, average age was 7.9 yrs, average hospitalization period was 4.5 days. 2. In simple X-ray studies, no bony change was detectable except for soft tissue signs in 68% of the cases. 3. Capsule-to-bone distance in sagittal ultrasonographic section revealed abnormal increase in 87.5% of the affected hip. 4. Average capsule-to-bone distance of affected hip joint was 7.50 mm, while that of the normal hip was 4.26 mm. 5. As shown in the above studies, ultrasonography can be considered good noninvasive technique in detection and follow-up of hip effusion. So, it is thought to be a valuable method in the diagnosis of transient synovitis of the hip in children.