In salt-losing congenital adrenal hyperplasia (CAH), continuous therapy with glucocorticoids and 9 alpha-fluorohydrocortisone (9 alpha-F) remains the golden rule. Previous reports showed a growth promoting effect of 9 alpha-F therapy. In addition, 9 alpha-F seemed to have a negligible glucocorticoid action. To confirm these facts, we analyzed the clinical data and the biological markers of control of therapy in two groups of patients with salt-losing CAH aged from 2 to 12 years: group I: before (time 0) and 6 months after the increase in 9 alpha-F dosage (time +6); group II: at time 0 and time +6 but without change in 9 alpha-F dosage. Groups were similar in terms of mean age, bone age and hydrocortisone dose. The mean dose of 9 alpha-F was 68.2 +/- 5.0 micrograms/m2/d at time 0 and was increased to 98.6 +/- 7.7 micrograms/m2/d at time +6 in group I; it remained similar in group II. In group I, height velocity decreased significantly from 8.1 +/- 0.6 at time 0 to 6.3 +/- 0.3 cm/yr at time +6 (p < 0.01) while in group II there was no significant change. In group I, plasma renin activity decreased from 10.4 +/- 1.6 at time 0 to 3.9 +/- 1.1 ng/ml/h at time +6 (p < 0.005) and showed no change in group II. These preliminary results suggest that careful monitoring of 9 alpha-F is essential to control a proper growth rate.