The aims of this comparative multicenter study of 67 girls with Turner syndrome (TS) on three different therapeutical regimens were, first, to evaluate the effect of either recombinant human growth hormone (GH) alone or in combination with the anabolic steroid oxandrolone (Oxa) on height velocity and on Turner-specific bone age (BA'TS) and, second, to estimate the gain in final height taking the age at the onset of treatment into account. The mean advancement of BA'TS in 2 years of treatment was 2.5 years/2 years in group 1 (low dose GH: 16 IU/m2/week), 2.8 years/2 years in group 2 (high dose GH: 28 IU/m2/week) and 3.3 years/2 years in group 3 (GH: 24 IU/m2/week + Oxa: 0.06 mg/kg/day) instead of the expected 2 years/2 years advancement in untreated girls with TS. On all treatment regimens the advancement of BA'TS was more pronounced in the younger girls. In many girls with a BA'TS below 9 years at the onset of treatment the increase in height did not outweigh the advancement in BA'TS, suggesting that starting growth-promoting treatment before 9 years would not be the best way to improve final height. In our opinion, the optimal age for starting growth-promoting therapy is at 9 years. A start at a younger age might have no advantage in regard of an ultimate gain in final height. On the other hand, therapy should not be delayed much after the age of 9 years giving the girls with TS the possibility to catch up substantially before estrogen treatment is initiated.