Human uterine leiomyomas exhibit increased IGF-I binding compared to myometrium, while both tissues show IGF-I gene expression. In this study we have examined the functional importance of these findings by testing the presence of IGF-I in 15 leiomyoma biopsies and in 18 myometrium biopsies and the capacity of smooth-muscle cells cultured from these tissues to react to IGF-I. The mean IGF-I peptide concentration in leiomyomas was 3 times higher than in myometrium. This resulted from increased IGF-I uptake in leiomyomas rather than from increased synthesis, as these tissues contain higher concentrations of type-I IGF receptors, as detected by immunohistochemistry, and equal levels of IGF-I mRNA. Blocking IGF-I transport with cytochalasin-B and with the type-I IGF receptor blocking antibody alpha IR3 in cultured cells induced decreased immunostaining intensity for IGF-I in most myometrium and leiomyoma cultures, indicating that the detected IGF-I is internalized. Depending on the culture conditions, IGF-I administration yielded increased survival or a higher proliferation rate in leiomyoma cultures than in myometrium cultures, indicating the increased importance of exogenous IGF-I for the growth of transformed smooth-muscle cells. We conclude that the increased concentrations of type-I IGF receptors in leiomyoma compared to myometrial smooth-muscle cells are functional with respect to the enhanced internalization of IGF-I and that they provide these tumor cells with a growth advantage compared to their normal counterparts.