Radiological estimates of gestational age in late pregnancy were compared with the gestational ages based on first trimester ultrasound crown-rump length measurements in a series of 125 patients. It was found that the 95 per cent confidence limits of a radiological estimate were +/- 3 1/2 weeks, that both distal femoral and upper tibial epiphyses were frequently present much earlier than 36 and 38 weeks respectively, and that in almost half of the growth-retarded babies in the series there was a bone-age discrepancy of three or more weeks. It is recommended that more liberal use is made of early ultrasound measurements for the assessment of gestational age.