In untreated acromegalic patients, mean serum level of insulin-like growth factor (IGF) is two times higher than in controls, and all patients have IGF values above the mean + 2 SD of the controls, while some of these patients have normal GH values. After treatment, clinical status correlates better with IGF than with GH. Thus, measurements of serum IGF can be helpful in the diagnosis of acromegaly and in assessment of the efficiency of therapy.