The infant's femoral head form and position relative to the acetabulum are the major parameters for the diagnosis of a hip joint dysplasia. The analysis can be performed using conventional ultrasound 2D-images, which virtually cut the hip joint in a specific plane. The joint's shape is scored by geometric measures taken from the image. The imaging in a defined plane is tedious and demands some experience. Furthermore the reporting of a spatial geometry through 2D-images tends to be subjective and incomplete. To consider the entire joint geometry, a 3D-data set of conventional ultrasound images is recorded along with the transducer movement. The femoral head is measured automatically in the spatially arranged images by fitting a virtual sphere, which approximates the femoral head's contours. A pilot study based on 20 3D-ultrasound data sets of hip joints of infants aging 2 days–12 weeks shows highly accurate coincidence of the automatically calculated diameter and centre parameters with those of control measurements, i.e. virtual spheres interactively placed and parametrized.