Context: Multiple endocrine neoplasia type 1 (MEN1) is a tumor syndrome of the parathyroid, endocrine pancreas, and anterior pituitary caused by mutations in the MEN1 gene on 11q13. Objective: The goal of this study was to determine the MEN1 mutation spectrum and detection rate among Swedish patients and identify which patient categories should be tested for MEN1 mutations. Design/Setting/Patients: DNA sequences and referral forms from patients referred to the Department of Clinical Genetics at Karolinska University Hospital, Sweden, for clinical MEN1 mutation screening were analyzed. The mutation status of 371 patients (including 200 probands) was ascertained, and the multiplex ligation-dependent probe amplification (MLPA) assay was evaluated for the detection of large deletions. Main Outcome Measure: The main outcome measure was MEN1 genotypes. Results: Forty-eight of 200 index cases (24%) shared 40 different mutations (18 novel). A total of 69% of all mutations resulted in a truncated protein. Two large deletions were detected by MLPA. A total of 94% of all MEN1 families had a mutation in the coding region of the MEN1 gene. A total of 6% of sporadic cases had MEN1 mutations. There was no correlation between severe disease and mutation type or location. Conclusions: A total of 4% of all mutations were large deletions, and MLPA is now included in our standard MEN1 mutation screening. Individuals with at least one typical endocrine tumour and at least one of the following: 1) a first-degree relative with a major endocrine tumor, 2) an age of onset less than 30 yr, and/or 3) multiple pancreatic tumors/parathyroid hyperplasia were most likely to harbor a mutation, thus these patients should be screened for MEN1 mutations. Copyright © 2007 by The Endocrine Society.