Ultrasonic measurement of the biparietal diameter is a frequently used technique in the assessment of fetal maturity. The popularity of this technique is based partly upon the claim that it can be performed with considerable presicion, but recently published work has not supported this claim. This paper describes a blind clinical trial in which the precision of a single biparietal measurement was determined under normal clinical conditions. The results show that this measurement can be determined with a precision better than +/- 1.6 mm (+/- 2 standard deviations). The clinical significance of this result is discussed.