Hereditary mixed polyposis is a genetically heterogeneous, autosomal dominant condition with adenomatous, hyperplastic and juvenile polyps. We conducted a comprehensive clinical evaluation of a large Ashkenazi Jewish family with this phenotype and performed extensive genetic testing. As seen in one previous report, a 40 kb duplication upstream of GREM1 segregated with the polyposis/colon cancer phenotype in this kindred. Our study confirms the association of GREM1 with mixed polyposis and further defines the phenotype seen with this mutation. This gene should be included in the test panel for all Jewish patients with mixed polyposis and may be considered in any Ashkenazi patient with unexplained hereditary colon cancer when mutations in other hereditary colon cancer genes have been ruled out.