Objective: To establish the cut-off values of GH measured by immunofluorometric assay, a more sensitive and specific assay, in normal prepubertal children and compare their values with those of proven GH-deficient patients. Methods: 30 normal children (20 males) and 26 patients with known causes of GH deficiency were submitted to the clonidine test and their GH values were compared. A powdered clonidine tablet (0.1 mg/m<sup>2</sup>) was given orally and blood samples for GH measurements were drawn at times –30, 0, 60, 90 and 120 min. Results: GH peak values presented a wide variation ranging from 1.7 to 25 µg/l (mean ± SD = 12.87 ± 5.8 µg/l) in the normal group. The cut-off values for the 5th and 10th percentiles of the distribution curve were 3.3 and 5.5 µg/l, respectively. In the GH deficiency group, maximum GH levels after clonidine stimulation ranged from <0.1 to 2.1 µg/l (0.56 ± 0.58 µg/l). Conclusions: The cut-off values obtained with the immunofluorometric method are lower than the ones obtained by radioimmunoassay. We suggest a cut-off value of 3.3 µg/l (5th percentile) that ensures 100% of sensitivity along with 93% of specificity to exclude the diagnosis of GH deficiency when using this immunofluorometric method.