Circulating insulin-like growth factor I (IGF-I) levels are stable throughout the day and correlate with pulsatile endogenous growth hormone (GH) secretion. A single measurement of IGF-I may, therefore, provide information on the GH secretory status of an individual. The clinical use of IGF-I determination in the evaluation of short stature in children suspected of GH deficiency will be discussed. Previous methodological problems have now mostly been overcome, and IGF-I can be determined in serum from healthy children, although concentrations exhibit large inter-individual variations relating to age, gender and pubertal maturation. These variations must be taken into account before IGF-I can be evaluated in a clinical situation, and before it can be stated whether a certain value is too low for a given age. The diagnostic sensitivities of IGF-I reported in relation to the outcome of GH provocative testing are variable (47–100%). This variability probably results from the use of different IGF-I assays, IGF-I reference ranges, and different GH testing procedures. When proper IGF-I assays and reference ranges are used, however, IGF-I determination results in valid diagnostic information regarding children exhibiting short stature. Thus, an IGF-I value should always be evaluated together with auxological information and GH testing results before a decision is taken on whether or not to start GH therapy.