A retrospective study of 643 cutaneous horns examined in our department between 1970 and 1989 revealed that 38.9% were derived from malignant or premalignant epidermal lesions, and 61.1% from benign lesions. Four main features were associated with premalignant or malignant histopathological change at the base of a cutaneous horn (base pathology). These were: (i) age--the mean age of the patients whose cutaneous horns showed premalignant or malignant base pathology was 8.9 years greater than the mean age of the patients where base pathology was benign (P less than 0.0005); (ii) sex--males were more likely to develop a cutaneous horn with (pre)malignant base pathology (P less than 0.001); (iii) site--over 70% of all (pre)malignant lesions were found on the nose, pinnae, backs of hands, scalp, forearms and the face; a cutaneous horn found at these sites was 2.1 times more likely to have derived from a (pre)malignant base than from any other part of the body (P less than 0.0001); (iv) geometry of the lesion--lesions with a wide base or a low height-to-base ratio were significantly more likely to show (pre)malignant base pathology. Of those solar keratoses giving rise to cutaneous horns, 8.9% also showed histological evidence of change to squamous cell carcinoma (SCC); this figure was not significantly different from the 6.2% of solar keratoses without horns that showed SCC change during the same period (1970-89).