OBJECTIVE The aim of this study is to evaluate the significance of nasal bone ossification as a marker for trisomy 21 at 11 to 14 weeks' gestation in an unselected obstetric population referred to our Centre. METHODS A total of 1906 consecutive fetuses undergoing nuchal translucency scan at 11 to 14 weeks' gestation were evaluated for the presence of hypoplasia/absence of nasal bone. The data obtained were correlated with fetal karyotype. RESULTS A successful view of the fetal profile was obtained in 1752 fetuses (91.9%). The nasal bone was hypoplastic/absent in 12 of 19 fetuses with chromosomal abnormalities. There were 10 cases of trisomy 21, in 8 of which hypoplastic/absent nasal bone was observed. Furthermore, absence of nasal bone was recorded in 24 of 1733 chromosomally normal fetuses. CONCLUSIONS Nasal bone evaluation may improve the detection of trisomy 21 in the first trimester in an unselected obstetric population. Although numerically limited, our experience confirms that delayed nasal bone ossification (hypoplasia/absence of nasal bone) is rarely observed in chromosomally normal fetuses (1.4%). An appropriate training of operators is mandatory in order to achieve an acceptable performance.