To evaluate the frequency of associated endocrine disorders, 26 patients with the Zollinger-Ellison syndrome were restudied in a uniform screening programme. The examinations were directed primarily against disorders in the pituitary, parathyroid, thyroid, adrenal cortex, and endocrine pancreas. Fourteen endocrine disorders were found in 10 patients (38%). Ten of these were newly diagnosed. Five patients had hyperparathyroidism, and in two of these a pituitary adenoma was also suspected. Two had pituitary adenomas, one was suspected of having a pituitary adenoma and one of having adrenocortical hypofunction, and one showed multiple hormone production with hypoglycaemia and flushing. It is concluded that most cases of associated endocrinopathies are not diagnosed until specifically looked for. In the Danish population of patients with the Zollinger-Ellison syndrome the prevalence is about 38%.