Forty-nine pubertal tall boys with a mean height prediction of 203.59 cm according to the Bayley-Pinneau (BP) method were treated prospectively with 500 mg testosterone-oenanthate every 2 weeks for a period of 6 months. Before therapy chronological age (CA) was 14.14 years and bone age (BA) 13.88 years using the Greulich-Pyle (GP) method. During therapy BA advanced by 1.37 years. It continued to accelerate during the 6 months following therapy with a mean delta BA/delta CA being 3.01 at 3 months and 2.24 at 6 months after therapy. The 6 months value was only slightly less than the delta BA/delta CA of 2.47 obtained during therapy. The reduction in adult height was 7.26 cm or 50.8% of the predicted further growth in 12 boys with a long-term follow up of 2.5 years. This is similar to the 51.6% or 9.63 cm observed in 50 boys with a long-term follow up after 14.25 month treatment until a BA of 17 years or more. It is concluded that in the majority of cases high-dose testosterone therapy in boys of tall stature can be limited to a 6-month treatment period. Reassessment of the height prediction after a 6-month interval without therapy should define those patients who have to resume treatment because of their remaining excessive growth potential.