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If longitudinal studies represent the more direct way to analyze modifications resulting from aging, often for obvious reasons cross-sectional samples can also be used for this purpose. But in this case differences between age groups cannot be identified with the effects of age only; they can also be attributed to the effects of a secular trend. It is the aim of this study to show, using four cephalic dimensions and a cross-sectional sample, the need to take into account the effect o f a secular trend when the age effect is studied. In 1985 a study of head and facial measurements of 182 Belgian men, aged 25-54 years, was carried out. A simple comparison between different age groups (F tests) revealed significant differences for headbreadth and facial breadths. However, no difference for head length was observed. The amplitude of the secular trend in the Belgian population is already known; thus an easy calculation is proposed to define the effects of the age factor. For head breadth the whole difference seems to be the result of the important decrease of secular trend, whereas the global increase observed for the bizygomatic and bigonial breadths results in part from the decrease resulting from the secular trend and in part from the increase resulting from the age effect.