Transient synovitis of the hip is non-specific inflammation and self limited condition, which is most common cause of painful limping in children under 10 years of age. Ultrasonography presents some merits, simple, rapid, non-invasive, low cost, repetitive to assess soft structures in and around the hip joint compared to other diagnostic methods. We prospectively studied 32 cases of unilateral transient synovitis at OPD from Jan. 1988 to Oct. 1989 using 5-7.5 MH, probe ultrasonography. 2-times serial check up in symptomatic and asymptomatic stage about capsule thickness and bone-capsule distance in 3 different positions of the both hips were performed for comparision. The following results were obtained; 1. Boys were 25 cases (78%) and 26 cases (83%) were between 3 and 7 years old of age. 2. Bone-capsule distance in ultrasonography revealed abnormal increase in 72%. 3. External rotation position showed wider bone-capsule distance than other position. 4. Capsule thickness was 3.3mm in diseased and 3.0mm in sound, 0.3mm decrease after treatment. 5. Bone-capsule distance was 4.5mm in symptomatic stage, 2.7mm in asymptomatic stage, 1.8mm decrease after treatment in averge.