Biopsies from 20 patients with erythema ab igne (EAI) were examined and compared with biopsies from the legs of 7 elderly control subjects. Epidermal thinning and flattening of the dermo-epidermal junction were seen in most of the tissues examined but were more prominent in the EAI biopsies. Basal cell degenerative change with vacuolation was frequently observed in the EAI specimens. Epidermal atypia amounting to preneoplastic change was observed in 4 EAI biopsies. The dermis was thinned and showed (a) marked oedema and connective tissue disruption and (b) accumulation of elastic staining material. Both haemosiderin and melanin were found within the dermis. The small blood vessels showed a striking abnormality in 7 patients and 2 control biopsies in that the endothelial cells were enlarged and the nuclei were hyperchromatic and irregular in size and shape. This change may be due more to stasis than to chronic heating.