The aims of this study were to evaluate the usefulness of ultrasonography as the primary imaging method for hip joints and to establish the limits of normal variation of ultrasound measurements. Five hundred sixty-six children aged 1-23 months who had been referred for clinical suspicion of developmental dysplasia of the hip (DDH) were examined. In the youngest infants, the percentage cover of the cartilaginous femoral head by the bony acetabular roof (femoral head cover, FHC) was measured. When the ossification center had appeared, the cover was measured indirectly by the distance from the lateral tangent to the ossification center to the lateral acetabular rim (lateral head distance, LHD). The FHC of normal hips increased with age; the lower normal limit (mean -2 SD) was 46% in the youngest infants and 52% at age 4-5 months. The LHD also increased with age; the upper normal limit (mean +2 SD) was 3 mm in patients aged < 1 year of age and 4 mm in the oldest patients. Radiographs were obtained when ultrasound examination showed pathological hips and the radiographic findings were in good accordance with the ultrasound findings. Ultrasonography based on measurements of the cover of the femoral head represents a rapid and reliable technique for evaluation of hips in children in their first and second year of life. If ultrasonography is used as the primary imaging method, radiography can be omitted in approximately 95% of patients referred for clinical suspicion of DDH, because their hip joints are normal.