['We investigated whether inhibition of oestrogen biosynthesis with the aromatase inhibitor, letrozole, during adolescence improves near-final height in boys with constitutional delay of puberty.', 'Seventeen boys with constitutional delay of puberty were randomized to receive testosterone (T) enanthate (1 mg/kg i.m.) every 4 weeks for 6 months in combination with placebo (Pl, n = 8), or the aromatase inhibitor letrozole (Lz, 2.5 mg/day orally) (n = 9), for 12 months. After treatment, patients were followed up until near-final height. Height discrepancy was calculated as near-final height minus mid-parental target height.', 'The primary end point was the difference in near-final height between the groups treated either with T + Pl or T + Lz. Secondarily, height discrepancy and gain in height standard deviation score (SDS) were analysed in both groups.', 'Boys treated with T + Lz reached a higher mean near-final height than did boys on T + Pl (175.8 vs. 169.1 cm, respectively, P = 0.04). In T + Lz-treated boys, mean near-final height did not differ from their mid-parental target height (175.8 vs. 177.1 cm, P = 0.38), whereas in T + Pl-treated boys, mean near-final height was lower than mid-parental target height (169.1 vs. 173.9 cm, P = 0.007). T + Lz-treated boys had a greater increment in height SDS over the pretreatment height SDS than T + Pl-treated boys (+1.4 SDS vs.+0.8 SDS, P = 0.03).'] Does treatment with the aromatase inhibitor letrozole during adolescence increase near-final height in boys with constitutional delay of puberty?