Plasma concentration of insulin-like growth factor type-I (IGF-I) was studied among the Mannis in Thailand to find a possible cause of their short stature. The Mannis are hunting and gathering indigenous tribal peoples living in Asian tropical rain forests. A total of 50 plasma specimens from three different Manni groups in southern Thailand were used in this study. The concentrations of acid-ethanol extract of plasma IGF-I were measured by radio-immunoassay. We found that (1) plasma concentration of IGF-I in the Mannis was low, (2) there was no adolescent spurt in IGF-I levels, and (3) the post adolescent plasma IGF-I level of the Manni was significantly lower than that of age-matched Japanese. Low IGF-I levels among the Mannis may account for their short stature.