The insulin-like growth factors I and II (IGF-I and -II) are polypeptides which play an important role in growth and development of the organism. In the present report we describe the detection of human IGF-I RNAs (both type Ia and type Ib) and IGF-II RNAs in benign (leiomyoma) and malignant (leiomyosarcoma) tumours from smooth muscle origin, using Northern blot hybridization analysis. In normal smooth muscle tissue of the uterus we found low levels of IGF-I RNAs only. In the tumours the same IGF-I RNA species were detected as in adult non-tumour tissues (uterus, liver). For transcription of the IGF-II gene in these tumours, two promoters are used which are expressed in fetal liver, but not in adult liver. The presence of IGF-I and IGF-II RNAs was also established by nucleotide sequence analysis of recombinant DNA clones isolated from cDNA libraries derived from two leiomyosarcomas. The nucleotide sequences of these cDNA clones, together covering the entire coding regions of IGF-Ia and IGF-II var RNA, predict that IGFs encoded by the tumour RNAs do not differ in amino acid sequence from the corresponding polypeptides isolated from serum. In those tissues containing IGF-I RNAs, IGF-I immunoreactivity was also demonstrated.