56 children with a clinical diagnosis of unilateral transient synovitis of the hip underwent bilateral sonographic assessment. On the anterior scan, the distance between the femoral neck and the fibrous joint capsule was measured. This distance, which we call the synovial capsular complex distance, was compared with age-dependent normal values. An increased distance was found in all 56 symptomatic hips (mean 10 mm, SD 1.8). This distance was also increased in 14 hips on the contralateral side (mean 8 mm, SD 1.6). An effusion was demonstrated in 53 symptomatic hips and in 8 hips on the contralateral side. These findings indicate that in one quarter of children with symptoms of unilateral transient synovitis the contralateral hip may have an increased synovial capsular complex distance due to synovial swelling or joint effusion, suggesting an asymptomatic synovitis. We therefore recommend a comparison of the synovial capsular complex distance on the symptomatic side with age-related normal values, in addition to a comparison with the asymptomatic hip.