The purpose of this work was to extend to long bones the study on the ossification of human fetal skeleton in relation to conceptual age by a quantitative methodological approach. Postero-anterior scans were performed on 29 dried fetal femora (from 11.5 weeks of conceptual age to term) by a Hologic QDR 1000 X-ray densitometer with Ultra-Hi-Resolution software. The results were expressed as bone mineral content (BMC, g) and bone mineral density (BMD, g/cm2). BMD was calculated on a rectangular area corresponding to the total length and minimum width of the shaft. This area was divided into five equal sections along its longitudinal axis in order to estimate the rate of ossification from a spatio-temporal point of view. Our data show that BMC has a high correlation with conceptual age during the whole prenatal life, increasing in the third trimester (r >/= 0. 96). During development, BMD is progressively less correlated with conceptual age (r = 0.95 in the first half of development, r = 0.68 in the second half), particularly according to a bidirectional gradient from the middle to the proximal and distal ends of the shaft. Our findings confirm the data obtained in our previous studies on the ossification of fetal human spines, and suggest an individual variability in bone density at term of development and particularly at the level of spongiosa, viz. in the areas mostly involved in architectural changes during the morphogenesis of the long bones.