SummaryReports of eighteen eases of seborrhoeic keratosis showing varying degrees of cellular atypicality have been collected. It is almost certain that many more cases are filed under diagnoses implying malignancy. In common with similar lesions reported previously by various authors as malignant transformation, these lesions showed the characteristic pattern of seborrhoeic keratosis with, in addition, pleiomorphism, cellular anaplasia and abnormal mitotic activity in some degree. The lesions occurred predominantly in sun‐exposed areas of patients who were over fifty years of age. Most lesions were of long duration. None of the lesions have shown any convincing evidence of malignant behaviour. It is suggested that the lesion should be regarded as an atypical seborrhoeic keratosis until more convincing proof of its malignant potential is established. Sun damage in ageing skin is suggested as being of possible aetiological significance in the development of epithelial cellular atypicality in seborrhoeic keratosi