OBJECTIVE To evaluate growth rate and adult height with recombinant growth hormone (GH) treatment in girls with Turner syndrome (TS) and predictors of their growth response. METHODS Data on girls with TS who were treated with GH in the National Cooperative Growth Study (NCGS) were evaluated. As of January 1997, there were 2798 girls with TS in the NCGS database, 2652 of whom had not previously received GH. Follow-up data on growth were available for 2475 subjects, and data on adult height were available for 622. RESULTS The average age of girls with TS at enrollment in the NCGS was 10.1 +/- 3.6 years. These patients had severely short stature compared with that of unaffected American girls (height, 118.5 +/- 16.5 cm; height standard deviation score [SDS], -3.1 +/- 0.9), but their heights were typical of those of American girls with TS (TS-specific height SDS, 0.01 +/- 0.9). Treatment with GH for an average duration of 3.2 +/- 2.0 years resulted in an increase in height SDS of 0.8 +/- 0.7 compared with unaffected girls and of 1.2 +/- 0.8 compared with TS standards. Growth rates increased from 4.0 +/- 2.3 cm/year before treatment to 7.5 +/- 2.0 cm/year after 1 year of treatment. Duration of treatment with GH was the strongest predictor of change in height SDS. After 6 to 7 years of treatment with GH, there was a cumulative change of 2.0 in mean height SDS. The 622 girls who reached adult height were older when they began taking GH. Their mean height gain over pre-GH projected height was 6.4 +/- 4.9 cm after 3.7 +/- 1.9 years of treatment. Their adult height was 148.3 +/- 5.6 cm. CONCLUSIONS Although the response to treatment with GH varied, it was associated with highly significant gains in growth and adult height in girls with TS. Duration of treatment with GH was the most important variable predicting adult height.