High resolution diagnostic ultrasound was assessed as a screening method for craniospinal anomalies during the second trimester of pregnancy in a population at low risk for neural tube defects (83,403 mothers). The effectiveness of the test was about 60% and the failure rate mainly due to late attendance. In a subgroup (9325) where the screening purposes were satisfactorily fulfilled, the detection rate (87%) was substantially greater. The significance of the results and the cost/benefit ratio, especially compared with serum alpha-feto protein screening services, are then discussed.