Diagnosing growth hormone deficiency in adults is difficult. Provocation tests are most commonly used for the diagnosis with the insulin-induced hypoglycemia test nowadays considered as the "gold standard". The role of IGF-I concentrations in diagnosing growth hormone deficiency in adults is under discussion. In 58 adult patients with proven growth hormone deficiency, the sensitivity and specificity of IGF-I concentrations in relation to growth hormone deficiency were evaluated. Reference values of plasma IGF-I were obtained from 53 healthy volunteers. Using a calculated cut-off concentration of 15 nmol/l we were able to demonstrate that IGF-I concentration is a reliable screening method for growth hormone deficiency. Using this cut-off point in a patient population younger than 40 years of age, sensitivity was 90% and specificity 89%. For patients exceeding the age of 40 years, sensitivity, specificity and positive predictive value were rather low, but the negative predictive value was as high as 90%, indicating that for patients over 40 years IGF-I concentrations above 15 nmol/l exclude growth hormone deficiency. In summary, under the age of 40 years measuring plasma IGF-I provides an useful tool to diagnose growth hormone deficiency, whereas above 40 plasma IGF-I values exceeding 15 nmol/l virtually exclude growth hormone deficiency.