Objective: This study was undertaken to investigate the impact of incorporating assessment of the nasal bone into first-trimester combined screening by fetal nuchal translucency(NT) thickness and maternal serum biochemistry. Study design: In this prospective combined screening study for trisomy 21, the fetal nasal bone was also examined and classified as present or absent. A multivariate approach was used to calculate patient-specific risks for trisomy 21 and the detection rate(DR) and false-positive rate(FPR) were estimated. We examined 2 screening strategies; first, integrated first-trimester screening in all patients and second, first-stage screening of all patients using fetal NT and maternal serum free fl-hCG and PAPP-A, followed by second-stage assessment of nasal bone only in those with an intermediate risk of 1 in 101 to 1 in 1000 after the firststage. Results: The nasal bone was absent in 113(0.6%) of the 20,165 chromosomally or phenotypically normal fetuses and in 87(62.1%) of the 140 fetuses with trisomy 21. With combined first-trimester NT and serum screening, the DR of 90%was achieved at a FPR of 5%. Inclusion of the nasal bone, either in all cases or in about 10%of the total in the 2-stage approach, halved the FPR to 2.5%. Conclusion: Inclusion of the nasal bone in first-trimester combined screening for trisomy 21 achieves a DR of 90%for a FPR of 2.5%. Objective: This study was undertaken to investigate the impact of incorporating assessment of the nasal bone into first-trimester combined screening by fetal nuchal translucency (NT) thickness and maternal serum biochemistry. Study design: In this prospective combined screening study for trisomy 21, the fetal nasal bone was also examined and classified as present or absent. A multivariate approach was used to calculate patient-specific risks for trisomy 21 and the detection rate (DR) and false-positive rate (FPR) were estimated. ; first, integrated first-trimester screening in all patients and second, first-stage screening of all patients using fetal NT and maternal serum free fl-hCG and PAPP-A, followed by second-stage assessment of nasal bone only in those with an Results: The nasal bone was absent in 113 (0.6%) of the 20,165 chromosomally or phenotypically normal fetuses and in 87 (62.1%) o f the 140 fetuses with trisomy 21. With combined first-trimester NT and serum screening, the DR of 90% was achieved at a FPR of 5%. Inclusion of the nasal bone, either in all cases or in about 10% of the total in the 2-stage approach, halved the FPR to 2.5%. Conclusion: Inclusion of the nasal bone in first-trimester combined screening for trisomy 21 achieves a DR of 90% for a FPR of 2.5%.