Ultrasonography has become accepted as a useful imaging modality in the early detection of developmental dysplasia of the hip (DDH). The purpose of this study was to investigate the extent to which ultrasonographic measurements of femoral head coverage correspond to the categories of hip maturity defined by Graf's angle alpha. The infants in this study (1,037 infants, 2,034 hips) were examined as part of an ultrasound screening program for detecting DDH. We found that femoral head coverage is positively correlated with alpha angle, and we also found upper and lower threshold values of femoral head coverage (51% and 39%), such that all hips having these values or beyond had mature or pathological development, respectively. For the detection of hips having mature development, this provided a specificity of 100% (by definition) and a sensitivity of 82.6%. For hips having pathological development, specificity was 100% and sensitivity was 79.2%.