Multiple endocrine neoplasia type one (MEN 1) is characterized by tumours of the parathyroid glands, pancreatic islet cells and the anterior pituitary and follows an autosomal dominant pattern of inheritance. We report identical twins born to a family known to have the MEN 1 syndrome. The twins were identical until puberty. The first twin underwent puberty normally; the second, however, suffered an early pubertal arrest and was subsequently found to have a prolactinoma. Both were also subsequently shown to have primary hyperparathyroidism. Genetic studies have since confirmed the twins identical for the affected haplotype and show that this is inherited from the father who also has MEN 1. The gene for MEN 1 has now been localized to the long arm of chromosome 11. The current hypothesis is that expression of the syndrome involves two separate genetic mutations. The first mutation is inherited and thus present in all cells but the tumour manifests itself in the endocrine tissue only after a second mutation that represents elimination of the normal allele. In the case described the twins are proven genetically identical. The marked phenotypic difference between the two must, by inference, represent a second somatic mutation and is further supportive evidence of the two-mutation model of tumour expression.