A role for insulin-like growth factors (IGF) in autocrine or paracrine growth stimulation of tumor cells has been proposed for tumors of different origins. We have studied IGF gene expression in human uterus smooth muscle (myometrium) and in a panel of benign (leiomyoma) and malignant (leiomyosarcoma) smooth muscle tumors. Using RNA transfer blot analysis we could demonstrate that in smooth muscle tissue and tumors IGF genes are differentially expressed. The mRNA species detected had the same size as reported for IGF mRNAs from other tissues. However, the abundance of the IGF gene transcripts varied from tissue to tissue. The amounts of IGF mRNAs detected in smooth muscle tumors were compared to the levels found in normal smooth muscle. The IGF-I gene was expressed at high levels in normal myometrium and in leiomyomas but appears to be repressed in leiomyosarcomas. Also the IGF-I peptide was detected in myometrium and in leiomyomas, but in leiomyosarcomas the level was substantially lower. The IGF-II gene was expressed at low levels in normal myometrium and leiomyomas but is activated in leiomyosarcomas. With increasing malignancy from the two major IGF-II mRNA species, 6.0 and 4.8 kilobases, in particular the 6.0-kilobase mRNA is produced at higher levels. In conclusion, these data suggest that for IGF-I a role in tumor cell growth is not likely, but probably IGF-II is involved in malignant smooth muscle tumor growth progression.