Histological, histochemical and immunohistochemical findings were studied in 40 cutaneous biopsies from 7 patients with Cowden's disease. Most facial biopsies showed a spectrum of trichilemmomas and related follicular malformations, including cylindrical trichilemmomas, lobulate trichilemmomas (14 lesions), and a keratinizing type sharing features with inverted follicular keratosis. One facial growth showed trichilemmomal changes without apparent follicular origin. Studies for common papilloma virus structural antigens were negative, apart from a typical common wart in one patient. Immunohistochemical studies in 6 facial trichilemmomas and acral keratoses, using a panel of anti-keratin antibodies, disclosed only abnormal differentiation with lack of large keratins in the lobulate trichilemmomas. Nine biopsies revealed a distinctive type of fibroma characterized by an organized pattern of interwoven fascicles of collagen bundles with a laminated or tortuous appearance, embedded in abundant mucin. A number of fibromas showed striking hyalinizalion; these may represent a second microscopic hallmark of Cowden's disease in addition to facial trichilemmomas.