We have previously demonstrated that gestational age can be estimated from a diagram in which the percentage of lipid-carrying cells (amniotic fluid cytology) and the creatinine concentration in the amniotic fluid are plotted. Using this diagram for the purpose of dating, a comparison was made with prediction of gestational age by early ultrasound (biparietal diameter, BPD). Dating by the two methods showed a high correlation (r = 0.72) in 53 samples from 48 pregnancies and the two methods were in agreement within 2 weeks in 94%. In one third of infants with a low birth weight for age, however, amniotic fluid tests overestimated gestational age in comparison with early BPD. It is suggested that this disparity may be due to a combination of influence of early growth retardation on the BPD and accelerated maturation accompanying intrauterine growth retardation. Prior to elective delivery in pregnancies with uncertain dates and when an early ultrasonic examination has not been performed dating by amniotic fluid tests is reliable and is particularly useful.