Uterine leiomyosarcomata, malignant smooth-muscle tumors that may arise from existing leiomyomata or directly from the myometrium, have been shown to oversecrete insulin-like growth factor II (IGF-II). It has been shown recently that the IGF-II gene (IGF2) is maternally imprinted in both mice and humans. Only the paternal allele of IGF2 is usually expressed, except in adult liver and in the central nervous system in which biallelic expression is seen. Recently, loss of IGF2 imprinting has been reported in Wilms' tumor and in several other malignancies, and it has been suggested that biallelic expression of the gene leads to overexpression of IGF-II peptide and increased mitogenic activity. Using the ApaI restriction enzyme polymorphism in exon 9 of IGF2, we examined the expression of IGF2 in five informative uterine leiomyosarcomata and found that IGF2 was expressed biallelically in two tumors. In contrast, five samples of normal myometrium and six informative benign uterine leiomyomata showed monoallelic expression of IGF2. The incidence of heterozygosity at this locus was markedly lower in patients with leiomyosarcoma (6%) than in the healthy population (42% heterozygous; P < 0.01). H19, a gene that is imprinted in normal tissues, was expressed monoallelically in all informative normal and neoplastic uterine tumors. In contrast, the IGF-II receptor gene was expressed from both alleles in all tissues examined, confirming the finding that the human IGF-II receptor gene is not imprinted in humans. This study provides further evidence that relaxation of IGF2 genomic imprinting occurs not only in childhood tumors but also in adult-onset tumors, and it suggests a novel epigenetic mechanism for oncogenesis throughout life.