Growth hormone deficiency leads to profoundly decreased growth velocity and, when untreated, to pituitary dwarfism. We evaluated growth evolution for one to four years under rGH therapy (0.07 IU/kg/day, subcutaneously) at seventeen idiopathic pituitary dwarves with isolated GH deficiency, 13 boys and 4 girls, with a wide span of age at therapy onset (between 4 and 24 years old). Diagnosis was set subsequent to at least two negative GH stimulation tests. All patients were pre-pubertal, with a bone age below 13 years (Grunlich and Pyle Atlas) but had normal thyroid and adrenal function. Patients were divided into two subgroups: early-diagnosed patients (12 patients younger than 14 at therapy onset) and late-diagnosed patients (5 patients, diagnosed at a chronological age of over 16 years). Growth velocity was significantly increased in the entire group, from 0.33+/−0.07 cm/month before therapy onset to 0.8+/−0.05 cm/month for the whole follow-up period (P<0.0005). Catch-up growth was maximal during the first year of therapy, with a velocity of 1.04+/−0.16 cm/month, which decreased subsequently. Both mean growth velocities for the whole follow-up period (0.99+/−0.08 vs 0.5+/−0.06 cm/month) and for the first year of therapy (1.33+/−0.13 vs 0.61+/−0.09 cm/month) were significantly higher at the early-diagnosed patients (P<0.01), despite present radiographic growth potential. Early therapy onset in isolated GH deficiency is therefore important not only because patients have a smaller height handicap to recuperate in order to enter the normal growth channel, but also as our data suggest - because growth cartilage seems to loose with age its reaction potential to GH administration in pre-pubertal patients. Our data show, nevertheless, that high-dose rGH therapy is still beneficial in older pre-pubertal GH deficient patients by significantly accelerating growth speed. GH dosage should be diminished to adult substitutive levels and puberty should be triggered therapeutically once growth ceases.