Little is known about the pathophysiology of sporadic adrenocortical tumors in adults. Because loss of heterozygosity at the 11p15 locus has been described in childhood tumors, particularly, in adrenocortical tumors, associated with the Beckwith-Wiedemann syndrome and because insulin-like growth factor-II (IGF-II) is a crucial regulator of fetal adrenal growth, we looked for structural analysis at the 11p15 locus and IGF-II gene expression in 23 sporadic adrenocortical adult tumors: 6 carcinomas (5 with Cushing's syndrome and 1 nonsecreting) and 17 benign adenomas (13 with Cushing's syndrome, 1 pure androgen secreting, and 3 nonsecreting). Twenty-one patients were informative at the 11p15 locus, and six (four carcinomas and two adenomas) of them (28.5%) exhibited 11p15 structural abnormalities in tumor DNA (five, an uniparental disomy and one, a mosaicism). In a single case that could be further studied, a paternal isodisomy was observed. Very high IGF-II mRNA contents were detected in seven tumors (30%; 5 of the 6 carcinomas and 2 of the 17 adenomas). They were particularly found in tumors with uniparental disomy at the 11p15 locus. Overall, a strong correlation existed between IGF-II mRNA contents and DNA demethylation at the IGF-II locus. These data show that genetic alterations involving the 11p15 locus were highly frequent in malignant tumors, but found only in rare adenomas. These results in combination with evidence for overexpression of IGF-II from the 11p15.5 locus suggest that abnormalities in structure and/or expression of the IGF-II gene play a role as a late event of a multistep process of tumorigenesis.