Actinic keratoses (AKs) are small scaly red areas of skin characterised histologically by dysplasia, a minority of which are thought to be precursors of squamous cell carcinoma (SCC), and which show a high frequency of regression. Surprisingly, in view of their benign clinical course, they show a high frequency of loss of heterozygosity (LOH) with a median loss of four loci with almost 20% of lesions showing loss of eight or more alleles, as well as frequent p53 mutation. Loss was common on 3p (31%), 9p (39%), 9q (22%), 13q (52%), 17p (64%) and 17q (46%), and allele loss correlated with dysplasia. Topological disturbance of p21WAF1/CIP1 expression correlated with allele loss but was also seen together with increased wild-type p53 expression and an increase in the fraction of cycling cells in the absence of allele loss or p53 mutation, and is likely to represent an early change. P21WAF1/CIP1 expression appeared independent of p53 status. The frequency of LOH in AKs exceeded that of (invasive) SCCs suggesting that the relation between the accumulation of genetic change and behaviour for non-melanoma skin cancer is not straightforward.