BACKGROUND Some experts maintain that (1) > 90% of patients with multiple endocrine neoplasia type 1 (MEN1) are first seen with hyperparathyroidism (HPTH) so that routine screening for other features is unnecessary and (2) MEN1 has > or = 94% penetrance by age 50 years. METHODS We constructed a regional registry of patients with or at risk for MEN1 and examined phenotypic profiles in 34 patients. MEN1 was defined as (1) endocrinopathy of 2 of the 3 principal related tissues (parathyroid, gastrointestinal endocrine, pituitary) or (2) 1 such feature plus a first-degree relative with MEN1. RESULTS The initial feature of MEN1 was HPTH in 50%, pituitary tumor in 18%, and gastrointestinal endocrine tumor in 32% of patients, with overall incidences of 82%, 65%, and 74%, respectively. HPTH developed by age 50 years in 73% of patients and by age 70 years in 83%. Penetrance of MEN1 at age 50 years was 82%. Associated features included renal (1) and rectal (1) cancer, malignant thymic carcinoid (1), and malignant pheochromocytoma (1). CONCLUSIONS Expression of MEN1 can vary considerably from established patterns. In our geographic region HPTH does not routinely precede other features of MEN1 and cannot be used to distinguish affected patients among those at risk. MEN1 can be inapparent until late in life and may be significantly underdiagnosed.