Gender, age adjusted, population based reference ranges are necessary to use insulin-like growth factor 1 (IGF-1) as a diagnostic marker or for therapeutic monitoring in growth hormone (GH) related diseases. The aim of the present study was to describe the serum IGF-1 distribution and to calculate age and gender specific reference values for Caucasian adult population. A representative sample of 1002 male and 1039 female, totally 2041 participants aged above 18 years old was examined. The subjects suffering from diabetes mellitus, renal diseases, liver diseases, cancer, or diseases of pituitary gland were excluded by medical history, physical examination, and laboratory tests. The subjects were not using any drug that could affect IGF-1 levels. Body mass index (BMI) >30 or <18 kg/m(2) were excluded. Serum IGF-1 concentrations were determined by immunoradiometric assay (IRMA). Serum IGF-1 concentrations were declined with age in both males and females after the age of 18. Males had significantly higher serum IGF-1 levels than females in the age groups 18-24, 50-69 (P < 0.05), but not in others (P > 0.05). The present study established age and gender specific reference ranges for serum IGF-1 levels calculated for Caucasian adult population with IRMA that could be used in medical practice.