To optimize growth hormone (GH) treatment in girls with Turner syndrome, two multicentre studies were carried out in The Netherlands: a frequency-response study (study 1) and a dose-response study (study 2). In study 1, 19 girls with Turner syndrome, aged 11 years or older, were treated with one or two daily injections of GH at a total dose of 6 IU/m2/day (0.067 mg/kg/day) and ethinyloestradiol given orally at a dose of 0.05 microg/kg/day. All the girls reached final height. The mean (+/-SD) gain in final height was not significantly different between the once- or twice-daily regimens (7.6 +/- 2.3 versus 5.1 +/- 3.2 cm, respectively). The mean final height attained was 155.5 +/- 5.4 cm. All the girls exceeded their adult height prediction. In study 2, 68 girls with Turner syndrome, aged 2-11 years, were randomized into three dosage groups: A, B and C. During the first year, the girls in all the groups received GH at a dose of 4 IU/m2/day (0.045 mg/kg/day), which group A continued to receive throughout the study. At the start of the second year, groups B and C were switched to a dose of 6 IU/m2/day, which the girls in group B continued to receive for the remainder of the study. At the start of the third year, the girls in group C were switched to a dose of 8 IU/m2/day (0.090 mg/kg/day) for the remainder of the study. After 7 years of GH treatment, height SDS (based on Turner syndrome and normal population references) increased significantly in all three groups, but significantly more in groups B and C compared with group A (p = 0.02 and p = 0.001, respectively). Predicted adult height increased significantly, without a significant difference between the three dosage groups. The mean final heights of 25 of the girls were 159.1, 161.8 and 162.7 cm for groups A, B and C, respectively.