The ultrasonographic distance between the hip joint capsule and femur was measured in 75 healthy adults. The mean distance was 5.1 (SD 0.7) mm. The shortest distance was 3.0 mm, the longest 7.0 mm. The difference between the right and left hip was 0.3 (SD 0.3, range 0-1.2) mm. No correlation was found between the ultrasonographic distance and the subjects' height, body weight, age or sex. The reproducibility of the measurement was good, and the interobserver correlation was 0.94. It is concluded that an ultrasonographic distance between the hip joint capsule and the femur of 7 mm or more, and a difference between the hips of 1 mm or more suggest an intracapsular effusion in the joint in adults.