Patients with the hamartomatous polyposis Peutz-Jeghers and familial juvenile polyposis syndromes are predisposed to colorectal cancer but lack early genetic alterations found in adenomatous premalignant lesions. We studied hamartomatous polyps for the expression of an early preneoplastic colorectal neoplasia risk marker also found in familial adenomatous polyposis patients. Retrospective, genetic, and hospital archival tissue immunohistochemistry using Adnab-9, a premalignant marker often found in Paneth-like cells (PCs), was performed on sections of polyps from eight patients with Peutz-Jeghers syndrome, eight patients with familial juvenile polyposis, and 36 hyperplastic polyp control sections. Anti-α-defensin 5 (AD5), a universal PC marker, was also used to label a subgroup of sections. Hamartomatous polyposis patients also underwent specific genetic analysis. Eighty-nine percent of Peutz-Jeghers syndrome polyps labeled with Adnab-9 compared with 63% for AD5; 88% of familial juvenile polyposis sections also labeled with Adnab-9. Of the 36 hyperplastic polyp sections, only four (11%) labeled with Adnab-9 and one (3%) with AD5.Adnab-9 labeling of PCs in the epithelial elements of hamartomatous colonic lesions of hereditary hamartomatous syndrome patients reflects the predisposition to colorectal cancer, further justifying early intervention strategies.