Two families with autosomal dominantly inherited desmoid tumors have recently been shown to have germline mutations at the 3' end of the APC gene. We subsequently identified an Amish family with autosomal dominantly inherited desmoid tumors. Genetic analysis performed on one family member, a 47-year-old man with multiple desmoid tumors and no colon polyps, revealed a protein truncating mutation in the middle of the APC gene. The truncating mutation is the result of a 337-bp insertion of an Alu I sequence into codon 1526 of the APC gene. The presence of a poly(A) tail at the 3' end of the insertion suggests that the Alu I sequence was inserted by a retrotranspositional event. Germline insertions of Alu I sequences have occasionally been reported to cause other genetic diseases including type I neurofibromatosis, hereditary site-specific breast cancer (BRCA2), and hemophilia B. However, this is the first report of a germline mutation of the APC gene resulting from an Alu I insertion.