The cephalic index of the fetal head was determined by the ultrasonic measurement of the occipitofrontal-biparietal diameter (OF-BPD) ratio in a pilot study of 53 patients at risk of Down's syndrome. The mean value of the OF-BPD ratio was 1.24, and a linear correlation between the occipitofrontal diameter (OFD) and the biparietal diameter (BPD) was observed. There was one case of Down's syndrome in this series. The fetal head was markedly brachycephalic with an OF-BPD ratio of 1.073. The results of this study suggest that when fetal brachycephaly is present, the probability of Down's syndrome is increased. Further evaluation of this parameter is warranted, as it is possible that this technique may provide a safe, and non-invasive method of screening the entire pregnant population for Down's syndrome.