Objectives: Investigation of small numbers of parathyroid tumours by X-chromosome inactivation analysis suggests that the majority of them are monoclonal lesions most likely caused by a somatic mutation. Somatic mutations in the MEN1 gene located on chromosome 11q13 have recently been identi®ed in 12±17% of solitary parathyroid tumours in patients with sporadic primary hyperparathyroidism, and they may be the precipitating genetic defect leading to monoclonal cell proliferation in these tumours. Design: To determine the prevalence of MEN1 gene mutations in monoclonal parathyroid neoplasias we investigated 33 parathyroid tumours of patients with primary hyperparathyroidism for clonality and mutations in the MEN1 gene. Methods: X-chromosome inactivation analysis was used to assess the clonal status of the tumours, direct sequencing of the complete coding region was applied to identify mutations in the MEN1 gene. Results: Twenty-eight female patients (26 patients with solitary adenoma, 2 patients with hyperplasia) were informative for the polymorphism of the androgen receptor on the X-chromosome and could be tested for inactivation pattern. Nineteen of twenty-six (73%) solitary adenomas were monoclonal. Somatic mutations in the MEN1 gene were identi®ed in nine cases. Six of them were found in the relatively large second exon of the MEN1 gene (A49D, 193del36, 402delC, 482del22, 547delT, W126X). One was found in exon 5 (904del9), one in exon 7 (Y327X) and one in exon 9 (R415X). Of the monoclonal tumours, 5 out of 19 (26%) harboured a somatic MEN1 gene mutation. Conclusions: In summary, 73% of the solitary parathyroid adenomas were monoclonal. In 26% of the monoclonal tumours a somatic MEN1 gene mutation has been identifed. However, for 74% of monoclonal tumours of the parathyroids the underlying genetic defects are still not known.