1689 people aged 40 years and over were examined over a 5-year period to determine the incidence of malignant transformation of solar keratoses. They were seen on 2 consecutive years on 4267 occasions; a total of 21 905 solar keratoses were present on the first visit. A squamous cell carcinoma (SCC) developed within 12 months on 28 of the 4267 occasions. Where accurate mapping of both SCCs and pre-existing solar keratoses was available, it was found that 10/17 (60%) SCCs arose from a lesion diagnosed clinically as a solar keratosis in the previous year and the other 7(40%) SCCs on what had been clinically normal skin 12 months previously. The risk of malignant transformation of a solar keratosis to SCC within 1 year was less than 1/1000. The cost-effectiveness of treating all solar keratoses to prevent the development of SCC is questionable.