During the past 2 years we have studied eighty-nine biopsy specimens from nineteen patients with Cowden's syndrome. Among fifty-three facial lesions, twenty-nine were trichilemmomas. Twenty-three of the others were consistent with trichilemmoma, showed a non-specific benign verrucous acanthoma, or were not diagnostic; one was a blue naevus. All fourteen oral mucosal biopsy specimens were benign fibromas. Nineteen of twenty-two biopsy specimens from the hands and feet showed the pattern of benign keratosis. Multiple trichilemmomas were found in all patients with Cowden's syndrome, but at times several biopsy specimens were required before a diagnostic picture was uncovered. All patients with multiple facial trichilemmomas were found to have Cowden's syndrome. The combination of multiple facial trichilemmomas, oral fibromas, and benign acral keratoses enables one to diagnose Cowden's syndrome at a stage before serious internal complications develop.