It is uncertain the Forced Expiratory Time (FET) acceptable during spirometry of young people (16-20 years old) because they are not children nor adults. We studied 235 boys and 187 girls of this age and, comparing spirometries with FET > 3 s and > 6 s, we found no significative difference (t test) between Vext, DtPEF, FVC and FEV1 repeatibility, while for wheight in males (p =0.0022) and plateau time (p < 0.00001) in males and females. We conclude that blows with FET < 6s but > 3s can be acceptable in young people when other criteria are respected.