Objective: Insulin-like growth factor-I (IGF-I) and IGF binding protein-3 (IGFBP-3) levels are important markers in diagnosis of growth hormone (GH) related disorders. The normal levels of IGF-I and IGFBP-3 vary among different ethnic groups, and using the references derived from different populations may sometimes be misleading during diagnosis, treatment and follow-up. We examined the levels of IGF-I and IGFBP-3 in healthy adult Turkish population. Material and Methods: Eight hundred and thirty-three subjects (512 females, 321 males) were enrolled in the study. Serum IGF-I and IGFBP-3 levels were measured by immunoradiometric assay in all participants. The study population was divided into age groups (18-20, 21-23, 24-25, 26-30, 31-40, 41-50, >50 years of age) and gender groups (females and males separately in the population <= 30 years of age, combined in age groups over 30 years of age) according to the references defined by the kit manufacturer and the results were compared to the reference values provided by the manufacturer that represents a reference population. Results: Serum IGF-I levels were statistically higher than the reference levels in all age groups of women s 30 years of age (p<0.05). In men, IGF-I levels were significantly higher (p<0.05) only in 26-30 years age group. In gender-combined groups over 30 years of age, IGF-I levels were statistically higher than the reference levels (p<0.05). Serum IGFBP-3 levels were significantly lower than the reference values in 24-25 years age group in both genders and in 18-20 years of age in males (p<0.05). Serum IGFBP-3 levels were significantly higher in 26-30 years age group in males and in all gender-combined groups >30 years of age (p<0.05). Conclusion: Serum IGF-I concentrations of our study population are generally higher than the reference values of the commercial kit. Centers dealing with GH disorders might benefit from defining their own population's normal values for IGF-I and IGFBP-3 to overcome possible diagnostic and follow-up pitfalls.