We studied height velocity (HV), bone age progression (delta BA/delta CA), urinary pregnanetriol (PT) and plasma 17-hydroxyprogesterone (17-OH-P) during the first years of life in 12 patients with 21-hydroxylase deficiency, treated by cortisone acetate. In the well-controlled phases normal growth rate (SDS between -1 and +1), satisfactory bone age progression (delta BA/delta CA < or = 1) and no clinical sign of poor treatment were found; in the undertreatment phases enhanced growth rate, rapid bone age progression and, in some instances, signs of virilization were found; in the overtreatment phases, reduced growth rate was the only sign of poor treatment. Hormonal values were only weakly correlated to therapeutic control. Therefore, growth rate evaluation can represent the best method of monitoring treatment in very young patients with 21-hydroxylase deficiency.